Remove ITERM appdef

bd9f34d added a custom ITERM appdef, because iTerm 2 and Terminal.app
use different keyboard shortcuts for splitting panes. However, that
commit had an unintended side effect [1]. 2e288a4 resolved those issues,
but it required a lot of additional configuration. (I'd rather not have
all that additional configuration if I can avoid it.)

Since I don't use Terminal.app, I don't really care if the split pane
keymapping (from bd9f34d) is broken in Terminal app. So, let's take a
simpler approach:

- Revert 2e288a4
- Remove the ITERM appdef
- Define the split pane keymapping using the default "TERMINAL" appdef.
  This will cause the keymapping to be active in both iTerm and
  Terminal.app. It won't work in Terminal.app, but that won't affect me.

[1]: https://groups.google.com/forum/#!topic/keyremap4macbook/C46suJM5kng