Remove Cmd-Shift-T Shortcut

Because:

- Users who wish to remap Cmd-Shift-T to Safari's "Undo Close Tab" menu item
  (providing equivalent, single-tab-reopen functionality) may do so by simply
  assigning a keyboard shortcut in System Preferences -> Keyboard -> Shortcuts

And because:

- There are other Safari extensions (like Antrikshy/RecoverTabs) which provide
  more comprehensive support for Cmd-Shift-T tab-reopening, including the
  ability to reopen more than one tab
- This extension's remapping of Cmd-Shift-T breaks other in-browser Safari
  plugins that attempt to use Cmd-Shift-T

This commit:

- Reverts rs/SafariTabSwitching#15 and removes support for reopening tabs using Cmd-Shift-T.