Remove InputMethodManagerDelegate#startInput()

This is a follow up CL to my previous CL [1], which partly reverted
the initial commit of ImeFocusController [2].

With that CL,

  InputMethodManagerDelegate#startInput()

is no longer called.  Thus we no longer need to defined it.

There must be no behavior change.

 [1]: Ie3446e1d0f62c489195e31fc7f073020884d6cae
      ff975479
 [2]: Ib455704fe1e9d243f93190a84f230210dbceac2a
      970d9d2e

Bug: 234882948
Bug: 244504062
Test: presubmit
Change-Id: I62ae648b75cc33592e4e985b4c975c97ccdd24f8