VideoCommon: Drop "Disable destAlpha" hack

This option has no use any more, neither performance nor driver workaround.

Source/Android/app/src/main/res/xml/preferences.xml

@@ -1030,12 +1030,6 @@
             <!-- Other Hacks -->
             <PreferenceCategory android:title="@string/other">
 
-                <CheckBoxPreference
-                    android:defaultValue="false"
-                    android:key="disableDestinationAlpha"
-                    android:summary="@string/disable_destination_alpha_descrip"
-                    android:title="@string/disable_destination_alpha"/>
-
                 <CheckBoxPreference
                     android:defaultValue="true"
                     android:key="fastDepthCalculation"
@@ -1066,4 +1060,4 @@
     </PreferenceCategory>
 
     <!-- TODO Add the About information here too. -->
-</PreferenceScreen>
+</PreferenceScreen>