(css) Drop unused "theme" Sass files

Only the original ng-material theme files are used in the build process
of ng-material (using gulp).

UI/WebServerResources/scss/components/subheader/subheader-theme.scss

@@ -1,14 +0,0 @@
-.md-subheader.md-THEME_NAME-theme {
-  color: '{{ foreground-2-0.23 }}';
-  background-color: '{{background-hue-3}}';
-
-  &.md-primary {
-    color: '{{primary-color}}'
-  }
-  &.md-accent {
-    color: '{{accent-color}}'
-  }
-  &.md-warn {
-    color: '{{warn-color}}'
-  }
-}