Bug 1823979 - Remove strict assertion in text selection. r=morgan

I was trying to anticipate all the conditions that would bring us
to a non-text leaf text point in a text selection. But it is hard to
account for other "correct" cases so this assertion should probably just
go away.

Differential Revision: https://phabricator.services.mozilla.com/D173760

accessible/base/TextLeafRange.cpp

@@ -566,7 +566,6 @@ MOZ_CAN_RUN_SCRIPT static std::pair<nsIContent*, int32_t> DOMPointForSelection(
       }
     }
 
-    MOZ_ASSERT(aPoint.mAcc->IsDoc() || content->HasFlag(NODE_IS_EDITABLE));
     return {content, 0};
   }