chore: Remove dead code due to unused arguments

- The skipResetBindings constructor parameter was removed in
  4ee29e89, so all related code can be
  removed.
- The ShortcutsEpic.openSidebarDropdown static method's only parameter
  was removed in
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75302, so
  there's no point passing anything to it.
- The ShortcutsTestCase.openSidebarDropdown static method never expected
  any parameters since creation in
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/109527, so
  there's no point passing anything to it.