Small fixes for AboutActivity

* Remove parent activity
 * Finish activity on "up action"
 * Remove unused dimensions

app/src/main/AndroidManifest.xml

@@ -212,11 +212,7 @@
         <activity
             android:name=".about.AboutActivity"
             android:label="@string/title_activity_about"
-            android:parentActivityName=".MainActivity"
             android:theme="@style/AppTheme">
-            <meta-data
-                android:name="android.support.PARENT_ACTIVITY"
-                android:value="org.schabi.newpipe.MainActivity" />
         </activity>
     </application>
 

app/src/main/java/org/schabi/newpipe/about/AboutActivity.java

@@ -94,6 +94,9 @@ public class AboutActivity extends AppCompatActivity {
         int id = item.getItemId();
 
         switch (id) {
+            case android.R.id.home:
+                finish();
+                return true;
             case R.id.action_settings:
                 NavigationHelper.openSettings(this);
                 return true;

app/src/main/res/values/dimens.xml

@@ -47,9 +47,6 @@
     <!-- Default screen margins, per the Android Design guidelines. -->
     <dimen name="activity_horizontal_margin">16dp</dimen>
     <dimen name="activity_vertical_margin">16dp</dimen>
-    <dimen name="app_bar_height">180dp</dimen>
-    <dimen name="fab_margin">16dp</dimen>
-    <dimen name="text_margin">16dp</dimen>
     <dimen name="appbar_padding_top">8dp</dimen>
 
 </resources>