Remove unnecessary check for transformer frequency tag

This check claims to come from JOSM but it seems to have been removed
from JOSM.

In general, we don't require that frequency is tagged on power objects
unless it differs from the standard grid frequency.

This also agrees with the [wiki
page](https://wiki.openstreetmap.org/wiki/Tag:power=transformer)

plugins/Power.validator.mapcss

@@ -43,12 +43,3 @@ node[power=transformer][voltage] {
     -osmoseItemClassLevel: "9100/91002/2";
     -osmoseTags: list("tag");
 }
-
-node[power=transformer][!frequency] {
-    throwWarning: tr("Power Transformers should have a frequency tag");
-    -osmoseItemClassLevel: "9100/91003/3";
-    -osmoseTags: list("tag");
-
-    assertMatch: "node power=transformer";
-    assertNoMatch: "node power=transformer frequency=50";
-}