Revert "event/SocketEvent: add assert()"

This reverts commit 7bc1c9925b.  It
caused a crash with the ALSA plugin family (through
MultiSocketMonitor::ReplaceSocketList() and
MultiSocketMonitor::AddSocket()).  Until we have a proper fix, the
assertion patch is reverted.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1020

src/event/SocketEvent.cxx

@@ -60,8 +60,6 @@ SocketEvent::Abandon() noexcept
 bool
 SocketEvent::Schedule(unsigned flags) noexcept
 {
-	assert((flags & IMPLICIT_FLAGS) == 0);
-
 	if (flags == GetScheduledFlags())
 		return true;