fix(option): revert duplicate prop (#3051)

src/lib/core/option/_option.scss

@@ -1,9 +1,6 @@
 @import '../style/menu-common';
 @import '../a11y/a11y';
 
-/** Default size of option text. */
-$mat-option-font-size: 16px !default;
-
 /**
  * This mixin contains shared option styles between the select and
  * autocomplete components.
@@ -14,7 +11,6 @@ $mat-option-font-size: 16px !default;
     position: relative;
     cursor: pointer;
     outline: none;
-    font-size: $mat-option-font-size;
 
     &[aria-disabled='true'] {
       cursor: default;