Drop ccache usage from pbuilder

We get random build failures like:

| 22:11:19 CC (gcc) [kamailio]		atomic_ops.o
| 22:11:19 ccache: FATAL: Failed to create /var/cache/pbuilder/ccache/5/3: Permission denied
| 22:11:19 make[1]: *** [atomic_ops.o] Error 1

This seems to be caused by some race condition (because
the directory permissions seem to be just fine), since
we don't have a persistent ccache directory anyway let's
just drop ccache usage (at least for now).