Remove unused UnbindReason.ACCESSIBILITY_SERVICE_DISABLED

UnbindReason.ACCESSIBILITY_SERVICE_DISABLED was introduced when we
implemented A11yIME [1], but it turns out that it has not been passed
to the IME client.

Let's remove it if it's not used.

 [1]: Ia651a811093a939d00c081be1961e24ed3ad0356
      fb17e5ae

Bug: 234882948
Test: presubmit
Change-Id: Ib6c011d4b5f6fb39f0bda84d51b73a21205ef0ca