don't map GDK_KEY_RELEASE to keyboard events, tdf#92996

This was only done in the new gtk3 backend, all other backends seem to
ignore the GDK_KEY_RELEASE event (especially the gtk2 one). So make the
gtk3 backend code consistent with the other backends.

Change-Id: I3bdecb7ce05190ee2496bc552ca79375fb6fd713
Reviewed-on: https://gerrit.libreoffice.org/17431Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/unx/gtk/app/gtkinst.cxx

@@ -138,7 +138,7 @@ static VclInputFlags categorizeEvent(const GdkEvent *pEvent)
         nType = VclInputFlags::MOUSE;
         break;
     case GDK_KEY_PRESS:
-    case GDK_KEY_RELEASE:
+    // case GDK_KEY_RELEASE: //similar to the X11SalInstance one
         nType = VclInputFlags::KEYBOARD;
         break;
     case GDK_EXPOSE: