cache.h: drop LOCAL_REPO_ENV_SIZE

cache.h: drop LOCAL_REPO_ENV_SIZE

We keep a static array of variables that should be cleared
when invoking a sub-process on another repo. We statically
size the array with the LOCAL_REPO_ENV_SIZE macro so that
any readers do not have to count it themselves.

As it turns out, no readers actually use the macro, and it
creates a maintenance headache, as modifications to the
array need to happen in two places (one to add the new
element, and another to bump the size).

Since it's NULL-terminated, we can just drop the size macro
entirely. While we're at it, we'll clean up some comments
around it, and add a new mention of it at the top of the
list of environment variable macros. Even though
local_repo_env is right below that list, it's easy to miss,
and additions to that list should consider local_repo_env.

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