http.c: remove unnecessary casts to long

http.c: remove unnecessary casts to long

When parsing 'http.lowSpeedLimit' and 'http.lowSpeedTime', we explicitly
cast the result of 'git_config_int()' to a long before assignment. This
cast has been in place since all the way back in 58e60dd203 (Add support
for pushing to a remote repository using HTTP/DAV, 2005-11-02).

But that cast has always been unnecessary, since long is guaranteed to
be at least as wide as int. Let's drop the cast accordingly.

Noticed-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>