Fix the new rolling boulder traps for when monsters step on them

The traps don't disappear when stepped on any more; that should be
true even when monsters step on them.

(This bug was reported privately to me via IRC, rather than via the
devteam's email address, so there isn't a ticket number to close.)

src/trap.c

@@ -2698,9 +2698,6 @@ trapeffect_rolling_boulder_trap(
                     trap->tseen = TRUE;
                 if (DEADMONSTER(mtmp))
                     trapkilled = TRUE;
-            } else {
-                deltrap(trap);
-                newsym(mtmp->mx, mtmp->my);
             }
             return trapkilled ? Trap_Killed_Mon : mtmp->mtrapped
                 ? Trap_Caught_Mon : Trap_Effect_Finished;