Remove show_ongoing_ime_switcher resource

In [1] we introduced a notification to enable switching across multiple IMEs,
together with the show_ongoing_ime_switcher resource, which was always
set to true. Later in [2] we fully removed the notification, but left
the resource and its usages. This can still block showing the IME Switcher
button in the navigation bar, if the resource is overridden to false.
However, that is not the intended usage of the resource. As this is
internal only, and otherwise always true, we can go ahead and remove it.

  [1]: I2b9e955003c1f6703978824c947d5f322a039714
  [2]: I3b09d2b05ba9bde686b360b9f720b5d4258551c3

Flag: EXEMPT cleanup
Bug: 409002606
Test: n/a
Change-Id: Idd93cf97590d3b6fbdf4cf3456bf137ca546af16