reduce amount of MAKEFLAGS cleaning

reduce amount of MAKEFLAGS cleaning

Now clean_jobserver_makeflags will only remove --jobserver settings
from MAKEFLAGS. This is simpler and easier to understand than
the old behavior, which, if there was no --jobserver, removed
all -j and --jobs, while leaving those when removing --jobserver.

This relies on -j options passed to make overriding
-j settings in MAKEFLAGS. So we don't need to clean those out,
we can just override them.