Remove Sass @euiFormControlDefaultShadow mixin usages (#194653)

## Summary

This PR removes the `euiFormControlDefaultShadow` mixin from Kibana
usage, which is shortly set to be deprecated/removed from EUI.

The usages of this mixin primarily wanted the `border` styling of the
mixin and not its background effects, so I've opted to simplify the CSS
greatly to simply use `border` CSS instead of attempting to mess around
with `box-shadow` (which wasn't really benefiting the final visual
appearance of the affected use cases).

I also incidentally removed some extra CSS specificity added in #156639
(no longer necessary as of #161592) which was causing exclusive borders
to not be the correct color.

| Before | After |
|--------|--------|
| <img width="696" alt=""
src="https://github.com/user-attachments/assets/00478e77-08e8-490d-90fa-84abd2d3ba99">
| <img width="704" alt=""
src="https://github.com/user-attachments/assets/46ef0a5f-5fb0-4d47-82ba-40ed7eb2ff89">
|

### Checklist

- [x] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [x] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

src/plugins/controls/public/control_group/components/control_panel.scss

@@ -216,7 +216,7 @@ $controlMinWidth: $euiSize * 14;
     align-items: center;
     border-radius: $euiBorderRadius;
     font-weight: $euiFontWeightSemiBold;
-    @include euiFormControlDefaultShadow;
+    border: $euiBorderWidthThin solid $euiFormBorderColor;
     background-color: $euiFormInputGroupLabelBackground;
     min-width: $controlMinWidth;
     @include euiFontSizeXS;
@@ -243,4 +243,4 @@ $controlMinWidth: $euiSize * 14;
       min-width: unset;
     }
   }
-}
+}

src/plugins/unified_search/public/filter_bar/filter_item/filter_item.scss

@@ -4,30 +4,26 @@
  * 1.  Allow wrapping of long filter items
  */
 
-.euiBadge.globalFilterItem {
+.globalFilterItem {
   line-height: $euiSize;
-  border: none;
   color: $euiTextColor;
-  padding-top: calc($euiSizeM / 2) + 1px;
-  padding-bottom: calc($euiSizeM / 2) + 1px;
+  padding-block: $euiSizeM / 2;
   white-space: normal; /* 1 */
 
   &:not(.globalFilterItem-isDisabled) {
-    @include euiFormControlDefaultShadow;
-    & { // stylelint-disable-line no-duplicate-selectors
-      box-shadow: #{$euiFormControlBoxShadow}, inset 0 0 0 1px $kbnGlobalFilterItemBorderColor; // Make the actual border more visible
-    }
+    border-color: $kbnGlobalFilterItemBorderColor; // Make the actual border more visible
   }
 }
 
-.euiBadge.globalFilterItem-isDisabled {
+.globalFilterItem-isDisabled {
   color: $euiColorDarkShade;
   background-color: transparentize($euiColorLightShade, .5);
+  border-color: transparent;
   text-decoration: line-through;
   font-weight: $euiFontWeightRegular;
 }
 
-.euiBadge.globalFilterItem-isError, .globalFilterItem-isWarning {
+.globalFilterItem-isError, .globalFilterItem-isWarning {
   .globalFilterLabel__value {
     font-weight: $euiFontWeightBold;
   }
@@ -47,6 +43,7 @@
 
 .globalFilterItem-isPinned {
   position: relative;
+  overflow: hidden;
 
   &::before {
     content: '';
@@ -56,14 +53,12 @@
     left: 0;
     width: $euiSizeXS;
     background-color: $kbnGlobalFilterItemBorderColor;
-    border-top-left-radius: calc($euiBorderRadius / 2);
-    border-bottom-left-radius: calc($euiBorderRadius / 2);
   }
 }
 
 .globalFilterItem-isExcluded {
   &:not(.globalFilterItem-isDisabled) {
-    box-shadow: #{$euiFormControlBoxShadow}, inset 0 0 0 1px $kbnGlobalFilterItemBorderColorExcluded;
+    border-color: $kbnGlobalFilterItemBorderColorExcluded;
 
     &::before {
       background-color: $kbnGlobalFilterItemPinnedColorExcluded;