Clean old symlink forests

There was an incrementality bug in the symlink forest generation that
was recently fixed. However, if the appropriate files don't get touched,
the issue will remain. Add a mechanism to clean old symlink forests
so that when we fix incrementality bugs we can ensure they don't remain
in any old out directories.

Bug: 276349152
Test: Presubmits
Change-Id: I36664dfb0ca7227e3e1f89de859ebccb808c5f15