fix(ui-shell): remove duplicate variable (#2728)

Author: tw15egan

packages/components/src/components/ui-shell/_theme.scss

@@ -115,12 +115,6 @@ $shell-panel-bg-04: $carbon--white-0; //TODO waiting for updated color
 /// @group ui-shell
 $shell-header-text-03: $carbon--gray-50; //TODO waiting for updated color
 
-/// Icons in header panel
-/// @type Color
-/// @access private
-/// @group ui-shell
-$shell-header-icon-02: #252525; //TODO waiting for updated color
-
 /// Header icon selected state background
 /// @type Color
 /// @access private