Android: Remove gradle.properties file handling

The modification of gradle.properties files was originally a feature of
the Android plugin which eventually moved over to androiddeployqt.

Moreover, this code got accidentally disabled in Qt Creator 4.10 and was
appartently not missed till now. I only noticed it while "FilePath-
ifying" code in the Andsroid plugin.

This change removes the gradle.properties file handling code from Qt
Creator.

Fixes: QTCREATORBUG-28494
Change-Id: Id4f2722f4965c877a065386b34a71d8368147b08
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

src/plugins/android/androidbuildapkstep.cpp

@@ -166,7 +166,6 @@ QWidget *AndroidBuildApkWidget::createApplicationGroup()
     connect(targetSDKComboBox, &QComboBox::activated, this, [this, targetSDKComboBox](int idx) {
        const QString sdk = targetSDKComboBox->itemText(idx);
        m_step->setBuildTargetSdk(sdk);
-       AndroidManager::updateGradleProperties(m_step->target(), QString()); // FIXME: Use real key.
    });
 
     auto formLayout = new QFormLayout(group);