oss-fuzz is finding possible fuzzing targets even under qemu-bundle/.../bin, but they
cannot be used because the required shared libraries are missing.  Since the
fuzzing targets are already placed manually in $OUT, the bindir and libexecdir
subtrees are not needed; remove them.

Cc: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>