Remove preprocessor guards from injection points

When defining an injection point there is no need to wrap the definition
with USE_INJECTION_POINT guards, the INJECTION_POINT macro is available
in all builds.  Remove to make the code consistent.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/OSCPR01MB14966C8015DEB05ABEF2CE077F51FA@OSCPR01MB14966.jpnprd01.prod.outlook.com
Backpatch-through: 17

src/backend/access/transam/xlog.c

@@ -7349,9 +7349,7 @@ CreateCheckPoint(int flags)
 	if (PriorRedoPtr != InvalidXLogRecPtr)
 		UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);
 
-#ifdef USE_INJECTION_POINTS
 	INJECTION_POINT("checkpoint-before-old-wal-removal", NULL);
-#endif
 
 	/*
 	 * Delete old log files, those no longer needed for last checkpoint to