bitbake.conf: Simplify CACHE setting

A long time ago the bitbake cache didn't use hashes in it's filename and
hence values such as MACHINE were needed in the path to the cache file
so that when switching MACHINE, a new cache wasn't always parsed.

Times have moved on, we have a hash which represents the configuration
and the caches are reused if there is an existing hash that matches.

This means the values added to CACHE are obsolete and not needed,
we can drop them.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>