Revert "Make `mapset_elem_of` Opaque."

This reverts commit 267cc708.

Thanks to @jung for spotting this was a NOP, see iris/stdpp!263 (comment 67379)

theories/mapset.v

@@ -132,8 +132,4 @@ Proof.
 Qed.
 End mapset.
 
-(** [mapset_elem_of] internally contains an equality; make sure that tactics do
-not unfold it and try to unify [∈] against goals with [=]. *)
-Opaque mapset_elem_of.
-
 Global Arguments mapset_eq_dec : simpl never.