This reverts commit 552c30ea.

It has caused various problems; for example, MPD wasn't able to write
the pid_file (which was already mitigated by commit a4e42172).

And apparently, the socket file created in the same directory by
mpd.socket disappears when mpd.service (re)creates the directory.  I
could not reproduce this problem with 247.3, but maybe this is a bug
in older systemd versions?

Until we figure out why this happens, let's remove the
RuntimeDirectory directive.  A future MPD version may be launched as
regular user, not as root, which will eliminate one major problem with
RuntimeDirectory.