siteconfig: Drop siteconfig class/code/support

The siteconfig code was only used for 5 cache values. The complexity added to sstate
to support this code was considerable and the runtime much more significant than
any benefit the cache files would have added. Drop the support for this which
was only used minimally for ncurses and zlib.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>