cred: proc_set_cred(): Remove obsolete comments

These have been obsoleted since they were first written (in commit
"cred: add proc_set_cred helper", daf63fd2f9a8d040, r280130) by some
subsequent commits ("proc: refactor clearing credentials into
proc_unset_cred", 5a90435ccfed8e1e, r361448; "cred: distribute reference
count per thread", 1724c563e62fa800, r361993).

No functional change.

Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46922

(cherry picked from commit 07c9edac7bcd75095724ff3c91d0aeec792a4f26)