Revert "Add recovery TAP test for race condition with slot invalidations"

This reverts commit 08a52ab151, due to some sporadic instability in
the test.  Getting the test right should require some redesign with a
second injection point, but let's revert it for now to avoid these
issues in the CI as a lot of patches are under discussion in this last
commit fest.

Per buildfarm members hachi and gokiburi.

Discussion: https://postgr.es/m/ZekQQHCrIqLVpGz5@paquier.xyz

src/backend/replication/slot.c

@@ -53,7 +53,6 @@
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/builtins.h"
-#include "utils/injection_point.h"
 
 /*
  * Replication slot on-disk data structure.
@@ -1659,14 +1658,6 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 				last_signaled_pid = active_pid;
 				terminated = true;
 				conflict_prev = conflict;
-
-				/*
-				 * This injection point needs to be after kill() to ensure
-				 * that the slot is not "active" anymore.  It also has to be
-				 * after ReportSlotInvalidation() to ensure that the
-				 * invalidation message is logged.
-				 */
-				INJECTION_POINT("terminate-process-holding-slot");
 			}
 
 			/* Wait until the slot is released. */