[Graph/SCSS] Migrate source_modal.scss file to css in js (#215168)

## Summary

Part of https://github.com/elastic/kibana/issues/208908

Replaces `source_modal.scss` files file to css-in-js.

### Checklist

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

x-pack/platform/plugins/private/graph/public/components/_index.scss

@@ -1 +0,0 @@
-@import './source_modal';

x-pack/platform/plugins/private/graph/public/components/_source_modal.scss

@@ -1,4 +0,0 @@
-.gphSourceModal {
-  width: 720px;
-  min-height: 530px;
-}

x-pack/platform/plugins/private/graph/public/components/source_modal.tsx

@@ -8,12 +8,13 @@
 import { EuiModalBody, EuiModalHeader, EuiModalHeaderTitle } from '@elastic/eui';
 import { FormattedMessage } from '@kbn/i18n-react';
 import React from 'react';
+import { css } from '@emotion/react';
 
 import { SourcePicker, SourcePickerProps } from './source_picker';
 
 export function SourceModal(props: SourcePickerProps) {
   return (
-    <div className="gphSourceModal">
+    <div css={sourceModalStyles}>
       <EuiModalHeader>
         <EuiModalHeaderTitle>
           <FormattedMessage
@@ -28,3 +29,8 @@ export function SourceModal(props: SourcePickerProps) {
     </div>
   );
 }
+
+const sourceModalStyles = css`
+  width: 720px;
+  min-height: 530px;
+`;

x-pack/platform/plugins/private/graph/public/index.scss

@@ -8,4 +8,3 @@
 //   gphChart__legend-isLoading
 
 @import './main';
-@import './components/index';