ceph.spec.in: remove build directory in %clean, not %install

ceph.spec.in: remove build directory in %clean, not %install

Removing the build directory at the end of %install is too soon,
and means we get rid of a bunch of stuff needed to correctly
create debuginfo/debugsource packages, which happens automatically
right after %install.  So, let's put it where it really belongs, in
the %clean section.

Fixes: aa18cb12003e3526c8e8f23dc2335a483fbfa68e
Fixes: https://tracker.ceph.com/issues/55079
Signed-off-by: Tim Serong <tserong@suse.com>
(cherry picked from commit 94ad178bdcbae56a8eafc65a3a276e25d7a51a5e)