vfs_shadow_copy2: use struct smb_filename.twrp instead of @GMT string token

vfs_shadow_copy2: use struct smb_filename.twrp instead of @GMT string token

Removes the asserts which were only needed to prove patchset correctness.

If smb_fname->twrp is zero, the path won't contain an @GMT token, so we can skip
the stripping logic and directly goto out.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>