ws: drop unused parameter from ws_blank_line()

ws: drop unused parameter from ws_blank_line()

We take a ws_rule parameter, but have never looked at it since the
function was added in 877f23ccb8 (Teach "diff --check" about new blank
lines at end, 2008-06-26). A comment in the function does mention how we
_could_ use it, but nobody has felt the need to do so for over a decade.

We could keep it around as reminder of what could be done, but the
comment serves that purpose. And in the meantime, it triggers
-Wunused-parameter.

So let's drop it, which in turn allows us to drop similar arguments
further up the callstack. I've left the comment intact. It does still
say "ws_rule", but that name is used consistently in the whitespace
code, so the meaning is clear.

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