Revert "systemd: add "RuntimeDirectory" directive"

This reverts commit 552c30eae4.

It has caused various problems; for example, MPD wasn't able to write
the pid_file (which was already mitigated by commit a4e4217204).

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.

systemd/system/mpd.service.in

@@ -7,10 +7,6 @@ After=network.target sound.target
 Type=notify
 ExecStart=@prefix@/bin/mpd --systemd
 
-# Create /run/mpd (if MPD is launched without the socket unit and is
-# configured to bind listener sockets there).
-RuntimeDirectory=mpd
-
 # Enable this setting to ask systemd to watch over MPD, see
 # systemd.service(5).  This is disabled by default because it causes
 # periodic wakeups which are unnecessary if MPD is not playing.