Revert "Add missing HTML imports for custom-style"

This reverts commit 40020b7a28.

Reason for revert:
Files app-theme.html and dark-theme.html are themes and can be loaded
dynamically. They are not nessecary to be vulcanized together with the
whole app and can be served as is (at present app-theme.html is
vulcnized inside gr-app and dark-theme is not). We should be able to
serve these files as is.

At the same time, polymer linter can't resolve custom-style tag, because
there is no link to it. Adding link to these files breake the abilit to
serve these files as is - instead they should be preprocessed. To solve
both tasks - avoid warings from polymer linter + serve files as is,
the files must not link any other files and polymer linter have to
ignroe it.

This change removes link to custom-style.html from the files and exclude
files from polymer linter.

Change-Id: I39da8cf0d257033e142bb0f47d78d3f77ed374a1

polygerrit-ui/app/polymer.json

@@ -4,7 +4,7 @@
     "behaviors/**/*",
     "elements/**/*",
     "scripts/**/*",
-    "styles/**/*",
+    "styles/*",
     "types/**/*"
   ],
   "lint": {

polygerrit-ui/app/styles/themes/app-theme.html

@@ -14,8 +14,6 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 -->
-<link rel="import" href="/bower_components/polymer/lib/elements/custom-style.html">
-
 <custom-style><style is="custom-style">
 html {
   /**

polygerrit-ui/app/styles/themes/dark-theme.html

@@ -14,8 +14,6 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 -->
-<link rel="import" href="/bower_components/polymer/lib/elements/custom-style.html">
-
 <dom-module id="dark-theme">
   <custom-style><style is="custom-style">
     html {