git-pull: dead code removal

git-pull: dead code removal

Back when a74b170 (git-pull: disallow implicit merging to detached HEAD,
2007-01-15) added this check, $? referred to the error status of reading
HEAD as a symbolic-ref; but cd67e4d (Teach 'git pull' about --rebase,
2007-11-28) moved the command away from where the check is, and nobody
noticed the breakage.  Ever since, $? has always been 0 (tr at the end of
the pipe to find merge_head never fails) and other case arms were never
reached.

These days, error_on_no_merge_candidates function is prepared to handle a
detached HEAD case, which was what the code this patch removes used to
handle.

Signed-off-by: Junio C Hamano <gitster@pobox.com>