CodingGuidelines: drop arithmetic expansion advice to use "$x"

CodingGuidelines: drop arithmetic expansion advice to use "$x"

The advice to use "$x" rather than "x" in arithmetric expansion was
working around a dash bug fixed in 0.5.4. Even Debian oldstable has
0.5.8 these days. And in the meantime, we've added almost two dozen
instances of the "x" form which you can find with:

  git grep '$(([a-z]'

and nobody seems to have complained. Let's declare this workaround
obsolete and simplify our style guide.

Helped-by: Danh Doan <congdanhqx@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>