upload-pack: remove superfluous sigchain_pop() call

upload-pack: remove superfluous sigchain_pop() call

2997178ee6 (upload-pack: split check_unreachable() in two, prep for
get_reachable_list(), 2016-06-12) moved most code of has_unreachable()
into the new function do_reachable_revlist().  The latter takes care to
ignore SIGPIPE during its operations, and restores the original signal
handler before returning.

However, a sigchain_pop(SIGPIPE) call remained in the error handling
code of has_unreachable(), which does nothing because the stack is
empty after do_reachable_revlist() cleaned up after itself.  Remove it.

Signed-off-by: René Scharfe <redacted>
Reviewed-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>