Remove final low-usage $euiHeader Sass variables + delete remaining…

… header scss files

- `$euiHeaderHeight` Sass variables have too much usage in Kibana etc., so keeping those for now - same for header Sass mixin

src/components/index.scss

@@ -10,7 +10,6 @@
 @import 'drag_and_drop/index';
 @import 'empty_prompt/index';
 @import 'form/index';
-@import 'header/index';
 @import 'key_pad_menu/index';
 @import 'markdown_editor/index';
 @import 'notification/index';

src/global_styling/variables/_header.scss

@@ -1,9 +1,3 @@
-// Themeable colors
-$euiHeaderBackgroundColor: $euiColorEmptyShade !default;
-
-// Layout vars
 $euiHeaderHeight: $euiSizeXXL + $euiSizeS !default;
-$euiHeaderChildSize: $euiSizeXXL !default;
-
 // Use the following variable in other components to afford for the fixed header
 $euiHeaderHeightCompensation: $euiHeaderHeight !default;

upcoming_changelogs/7005.md

@@ -12,3 +12,5 @@
 - Converted `EuiHeaderAlert` to Emotion; Removed unused `.euiHeaderAlert__dismiss` CSS
 - Converted `EuiHeaderSection`, `EuiHeaderSectionItem`, and `EuiHeaderSectionItemButton` to Emotion
 - Converted `EuiHeaderLinks` and `EuiHeaderLink` to Emotion; Removed `$euiHeaderLinksGutterSizes` Sass variables
+- Removed `$euiHeaderBackgroundColor` Sass variable; use `$euiColorEmptyShade` instead
+- Removed `$euiHeaderChildSize` Sass variable; use `$euiSizeXXL` instead