doc/git-branch: remove obsolete "-l" references

doc/git-branch: remove obsolete "-l" references

The previous commit switched "-l" to meaning "--list", but a
few vestiges of its prior meaning as "--create-reflog"
remained:

  - the synopsis mentioned "-l" when creating a new branch;
    we can drop this entirely, as it has been the default
    for years

  - the --list command mentions the unfortunate "-l"
    confusion, but we've now fixed that

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>