Cleanup build.gradle defaults not used

- remove defaultTargetSdkVersion which was never used
- comment out defaultBuildToolsVersion & defaultCompileSdkVersion
  with notes that these values should be defined and used
  someday in the future

Resolves apache#658
Resolves apache#631

bin/templates/project/build.gradle

@@ -40,10 +40,13 @@ allprojects {
 
     //This replaces project.properties w.r.t. build settings
     project.ext {
-      defaultBuildToolsVersion="28.0.3" //String
+      // FUTURE TBD
+      // (see <https://github.com/apache/cordova-android/issues/659>):
+      // defaultBuildToolsVersion="28.0.3" //String
       defaultMinSdkVersion=19 //Integer - Minimum requirement is Android 4.4
-      defaultTargetSdkVersion=28 //Integer - We ALWAYS target the latest by default
-      defaultCompileSdkVersion=28 //Integer - We ALWAYS compile with the latest by default
+      // FUTURE TBD
+      // (should be part of solution to <https://github.com/apache/cordova-android/issues/660>):
+      // defaultCompileSdkVersion=28 //Integer - We ALWAYS compile with the latest by default
     }
 }