[css-ui-3][css-ui-4][editorial] Incorporated @frivoal's feedback

Removed `hidden` and their title from the manual tests for "Cursor of the canvas" and removed tests for the non-standard property `user-modified`, as they will be marked as tentative.

Author: SebastianZ

css-ui-3/Overview.bs

@@ -1579,7 +1579,7 @@ but still generate boxes. For example,
 if the element has ''visibility: hidden'' but not ''display: none'',
 boxes are generated for it and its cursor is used for the canvas.
 
-<wpt hidden title="Manual tests">
+<wpt>
 canvas-cursor-001.html
 canvas-cursor-002.html
 </wpt>
@@ -2993,15 +2993,3 @@ parsing/field-sizing-computed.html
 parsing/field-sizing-invalid.html
 parsing/field-sizing-valid.html
 </wpt>
-
-<wpt hidden title="Tests for non-standard features">
-historical/moz-user-modify-01.html
-historical/moz-user-modify-02.html
-historical/moz-user-modify-03.html
-historical/user-modify-01.html
-historical/user-modify-02.html
-historical/user-modify-03.html
-historical/webkit-user-modify-01.html
-historical/webkit-user-modify-02.html
-historical/webkit-user-modify-03.html
-</wpt>

css-ui-4/Overview.bs

@@ -4534,15 +4534,3 @@ Appendix D. Default style sheet additions for HTML</h2>
 	text-overflow-string-005.html
 	text-overflow.html
 	</wpt>
-
-	<wpt hidden title="Tests for non-standard features">
-	historical/moz-user-modify-01.html
-	historical/moz-user-modify-02.html
-	historical/moz-user-modify-03.html
-	historical/user-modify-01.html
-	historical/user-modify-02.html
-	historical/user-modify-03.html
-	historical/webkit-user-modify-01.html
-	historical/webkit-user-modify-02.html
-	historical/webkit-user-modify-03.html
-	</wpt>