build: clean up only known subdirectories in build/*
build: clean up only known subdirectories in build/*

We remove the whole "build" directory while cleaning source code tree
because Gradle can leave there files even after "gradle clean". But some
projects (Mozilla Fennec) actually have useful stuff checked into VCS
under the "build" directory.

Remove only those subdirectories that we known for sure are leftovers
from Gradle.

Fixes fdroid/fdroidserver#438.

f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver