Remove useless "return;" lines

Discussion: https://postgr.es/m/20191128144653.GA27883@alvherre.pgsql

src/backend/access/gist/gist.c

@@ -1088,8 +1088,6 @@ gistFindCorrectParent(Relation r, GISTInsertStack *child)
 		LockBuffer(child->parent->buffer, GIST_EXCLUSIVE);
 		gistFindCorrectParent(r, child);
 	}
-
-	return;
 }
 
 /*

src/backend/access/rmgrdesc/genericdesc.c

@@ -43,8 +43,6 @@ generic_desc(StringInfo buf, XLogReaderState *record)
 		else
 			appendStringInfo(buf, "offset %u, length %u", offset, length);
 	}
-
-	return;
 }
 
 /*

src/backend/access/transam/twophase.c

@@ -2436,6 +2436,4 @@ PrepareRedoRemove(TransactionId xid, bool giveWarning)
 	if (gxact->ondisk)
 		RemoveTwoPhaseFile(xid, giveWarning);
 	RemoveGXact(gxact);
-
-	return;
 }

src/backend/access/transam/xact.c

@@ -3428,7 +3428,6 @@ CheckTransactionBlock(bool isTopLevel, bool throwError, const char *stmtType)
 	/* translator: %s represents an SQL statement name */
 			 errmsg("%s can only be used in transaction blocks",
 					stmtType)));
-	return;
 }
 
 /*