Remove unnecessary parentheses in return statements

The parenthesized style has only been used in a few modules.  Change
that to use the style that is predominant across the whole tree.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
Reviewed-by: Ryan Murphy <ryanfmurphy@gmail.com>

contrib/btree_gist/btree_utils_num.c

@@ -296,7 +296,7 @@ gbt_num_consistent(const GBT_NUMKEY_R *key,
 			retval = false;
 	}
 
-	return (retval);
+	return retval;
 }