Makefile: delete __pycache__ dirs in make clean

Move __pycache__ directory removal from debian/rules to the clean Makefile target. The change should have no effect on the generated packages; it just makes make clean more complete and debian/rules simpler.

Instead of using a shell loop for passing the files to rm, also use find's -delete option, avoiding any potential issues with weird filenames.

Since the __pycache__ folders may contain .pyc files, make sure these are all deleted before.