gitweb: simplify gitweb.min.* generation and clean-up rules

gitweb: simplify gitweb.min.* generation and clean-up rules

GITWEB_CSS and GITWEB_JS are meant to be "what URI should the installed
cgi script use to refer to the stylesheet and JavaScript", never "this
is the name of the file we are building".  Don't use them to decide what
file to build minified versions in.

While we are at it, lose FILES that is used only for "clean" target in a
misguided way.  "make clean" should try to remove all the potential
build artifacts regardless of a minor configuration change. Instead of
trying to remove only the build product "make clean" would have created
if it were run without "clean", explicitly list the three potential build
products for removal.

Tested-by: Mark Rada <marada@uwaterloo.co>
Signed-off-by: Junio C Hamano <gitster@pobox.com>