Fix Search Profiler's 'Index' input's placeholder color contrast accessibility. (#38952) (#39015)

x-pack/plugins/searchprofiler/public/app.js

@@ -65,6 +65,7 @@ function profileVizController($scope, $http, HighlightService, Private) {
   $scope.description = 'Search profiling and visualization';
   $scope.profileResponse = [];
   $scope.highlight = HighlightService;
+  $scope.index = '_all';
 
   // TODO this map controls which tab is active, but due to how
   // the tab directive works, we cannot use a single variable to hold the state.

x-pack/plugins/searchprofiler/public/templates/index.html

@@ -61,7 +61,6 @@
               class="kuiTextInput"
               type="text"
               ng-model="index"
-              placeholder="_all"
               ng-disabled="!licenseEnabled"
             ></input>
           </div>