android: Remove buildToolsVersion

Finally a default is configured and we don't have to update this
constantly.

src/frontends/android/app/build.gradle

@@ -2,7 +2,6 @@ apply plugin: 'com.android.application'
 
 android {
     compileSdkVersion 26
-    buildToolsVersion '27.0.3'
 
     defaultConfig {
         applicationId "org.strongswan.android"