Fix recent builds with bad dangling symlinks

It appears that there's a bug in repo where when a linkfile is removed
from the manifest, the symlink remains in the source tree.

Cleaning this up needs to happen before we start scanning the source
tree for Android.bp/Android.mk files, so this has to be done very early
-- way before CleanSpec processing could handle it.

Bug: https://bugs.chromium.org/p/gerrit/issues/detail?id=10583
Test: ln -s missing hardware/qcom/sdm710/Android.bp; m
Change-Id: Ib68f2507ffe58ccdd9fbc88925f8a4f6150f2f7d