Remove ill-conceived Assertion in ProcessClientWriteInterrupt().

It's perfectly fine to have blocked interrupts when
ProcessClientWriteInterrupt() is called. In fact it's commonly the
case when emitting error reports. And we deal with that correctly.

Even if that'd not be the case, it'd be a bad location for such a
assertion. Because ProcessClientWriteInterrupt() is only called when
the socket is blocked it's hard to hit.

Per Heikki and buildfarm animals nightjar and dunlin.

src/backend/tcop/postgres.c

@@ -571,8 +571,6 @@ ProcessClientWriteInterrupt(bool blocked)
 {
 	int			save_errno = errno;
 
-	Assert(InterruptHoldoffCount == 0 && CritSectionCount == 0);
-
 	/*
 	 * We only want to process the interrupt here if socket writes are
 	 * blocking to increase the chance to get an error message to the