ls-remote.txt: delete unsupported option

ls-remote.txt: delete unsupported option

-u <exec> has never been supported, but it was mentioned since
0a2bb55 (git ls-remote: make usage string match manpage -
2008-11-11). Nobody has complained about it for seven years, it's
probably safe to say nobody cares. So let's remove "-u" in documents
instead of adding code to support it.

While at there, fix --upload-pack syntax too.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>