build, qt: No need to set inapplicable QPA backend for Android

src/qt/android/src/org/bitcoincore/qt/BitcoinQtActivity.java

@@ -18,12 +18,6 @@ public class BitcoinQtActivity extends QtActivity
             bitcoinDir.mkdir();
         }
 
-        try {
-            Os.setenv("QT_QPA_PLATFORM", "android", true);
-        } catch (ErrnoException e) {
-            e.printStackTrace();
-        }
-
         super.onCreate(savedInstanceState);
     }
 }