Fix issue merging bp2build files with handcrafted ones

It was possible for the merged content to end up back in the bp2build
generated file because there was a symlink from the symlink forest to
the bp2build generated file.

Remove the symlink if it exists.

Bug: 276349152
Test: m bp2build, add a handcrafted file in the same folder as a Android.bp file, m bp2build again, check that the symlink forest version is not a symlink
Change-Id: Id64aa3addebcf0c6b1728389f21ae246796aaf8d