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 [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.

Your choice, of course, whether to merge this. Like [other users](https://github.com/rs/SafariTabSwitching/commit/e4e16cd08da9321be2ac00992647ce29e16312cd#commitcomment-7517551), I prefer to use a different extension for Cmd-Shift-T, and I can't use your extension with it.

[RecoverTabs]: https://github.com/Antrikshy/RecoverTabs
[assigning a keyboard shortcut]: http://support.apple.com/kb/ph13916