atmos: remove a few force-deletes in merge code

Removes del(giver) from pipeline/merge(), turf/assume_air() and
atom/assume_air().
Thanks to valzargaming on reddit/github for pointing it out.
fixes #4717

Signed-off-by: Mloc-Argent <colmohici@gmail.com>
Merge pull request #4719 from Mloc/bs12-atmg

atmos: remove a few force-deletes in merge code