Revert "EmscriptenApplication: get correct keyboardListeningElement CSS selector"

Not yet sure about all interactions with older Emscripten versions and
DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR set to 0, reverting until
it's clear.

This reverts commit c760acb548.

src/Magnum/Platform/EmscriptenApplication.cpp

@@ -517,7 +517,7 @@ void EmscriptenApplication::setupCallbacks(bool resizable) {
         if(element === document) return 1;
         if(element === window) return 2;
         if('id' in element)
-            return allocate(intArrayFromString('#' + element.id), 'i8', ALLOC_NORMAL);
+            return allocate(intArrayFromString(element.id), 'i8', ALLOC_NORMAL);
 
         return 0;
     }));
@@ -528,7 +528,7 @@ void EmscriptenApplication::setupCallbacks(bool resizable) {
         if(element === document) element = {id: '#document'};
         if(element === window) element = {id: '#window'};
         if('id' in element)
-            return allocate(intArrayFromString('#' + element.id), 'i8', ALLOC_NORMAL);
+            return allocate(intArrayFromString(element.id), 'i8', ALLOC_NORMAL);
 
         return 0;
     }));