remove "force_build_tools" config option

The force_build_tools config option was added a long time ago to brute force the build-tools version by trying to replace the value in build.gradle files. This is never something that should be used in production, since the app's build metadata should specify this kind of thing. And now that we're moving towards androguard for everything except fdroid build and fdroid publish, build-tools will no longer even be used in the other commands.

I propose we simplify the code and just remove it. Then if it is set in config.py there could be a error message saying it was removed. People who want it can still use the stable 1.1.x branch.