Properly closes #469: Don't use `yas--init-minor-keymap'

Previous reverted commit 498cbe4 forgot to take into account that one
of the tests used the `yas--init-minor-keymap' function that was
removed.

* yasnippet.el (yas-use-menu): Don't allow `nil' value, it's
useless.
(yas--minor-mode-menu): Use top-level `easy-menu-define' form.
(yas--init-minor-keymap): Remove definition.

* yasnippet-tests.el (test-rebindings): Don't use
`yas--init-minor-keymap'. Restore bindings explicitly. Not ideal,
but should work.