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.