fix(button): icon in button size

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/components/button/_button-theme.scss

@@ -376,20 +376,13 @@
         compact: rem(4px)
     );
 
-    $icon-sizes: map.get((
-        material: rem(18px),
-        fluent: rem(16px),
-        bootstrap: rem(18px),
-        indigo-design: rem(18px),
-    ), $variant);
-
     $filtering-row-button-size: (
         comfortable: rem(40px),
         cosy: rem(30px),
         compact: rem(21px)
     );
 
-    $icon-in-button-size: $icon-sizes;
+    $icon-in-button-size: rem(18px);
 
     $raised-shadow: map.get((
         material: var-get($raised-theme, 'resting-shadow'),