purge unneeded 'build_tools' config option

Back when fdroidserver was built around aapt, that was needed to
guarantee that a compatible version of aapt was used. Now, aapt is
only optionally used for getting the APK ID, so this was just
complicating maintenance.

tests/vcs.TestCase

@@ -59,7 +59,6 @@ class VCSTest(unittest.TestCase):
         os.mkdir("build")
         config['sdk_path'] = 'MOCKPATH'
         config['ndk_paths'] = {'r10d': os.getenv('ANDROID_NDK_HOME')}
-        config['build_tools'] = 'FAKE_BUILD_TOOLS_VERSION'
         config['java_paths'] = {'fake': 'fake'}
         fdroidserver.common.config = config
         app = fdroidserver.metadata.App()