bitbake.conf: Drop PE and PR from WORKDIR and STAMP

Once, we relied upon stamps changing to rebuild. PE and PR are packaging
variables and are reflected in the taskhashes when/where they're used so
they do not need to be in STAMP.

Similarly, once, multiple versions were something which was useful in
WORKDIR but this is really just noise causing long pathnames now.

Drop PR and PE from these variables to clean up the paths a bit.

This may break some tool assumptions about paths but those are probably
things we need to fix.

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