chore(form-field): remove dark theme (#2146)

packages/mdc-form-field/README.md

@@ -76,11 +76,6 @@ successive sibling is a `label` element. This means it will work for MDC Web for
 `mdc-form-field` is automatically RTL-aware, and will re-position elements within an RTL context.
 `mdc-form-field` will apply RTL styles whenever it, or its ancestors, has a `dir="rtl"` attribute.
 
-### Theming
-
-`mdc-form-field` is dark theme aware, and will change the text color to the "primary on dark" text
-color when used within a dark theme.
-
 
 ## JS Usage
 

packages/mdc-form-field/mdc-form-field.scss

@@ -23,16 +23,12 @@ $mdc-form-field-item-spacing: 4px;
 /* stylelint-disable selector-max-type */
 .mdc-form-field {
   @include mdc-typography(body1);
-  @include mdc-theme-prop(color, text-primary-on-light);
+  @include mdc-theme-prop(color, text-primary-on-background);
 
   display: inline-flex;
   align-items: center;
   vertical-align: middle;
 
-  @include mdc-theme-dark {
-    @include mdc-theme-prop(color, text-primary-on-dark);
-  }
-
   > label {
     order: 0;
     margin-right: auto;