(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/toast/toast-theme.scss

@@ -1,18 +0,0 @@
-md-toast.md-THEME_NAME-theme {
-  background-color: '{{foreground-1}}';
-  color: '{{background-50}}';
-
-  .md-button {
-    color: '{{background-50}}';
-  }
-
-  .md-action {
-    color: '{{primary-A200}}';
-    &.md-accent {
-      color: '{{accent-A200}}';
-    }
-    &.md-warn {
-      color: '{{warn-A200}}';
-    }
-  }
-}