Remove isMultiClientImeEnabled from SystemUI

The Multi-Client IME was removed by CL[1].

We no longer need the the isMultiClientImeEnabled
related logic in SystemUI. Remove it for code health.

[1]: I0bdc8fe3d32ccabc8ea7996fc689543c3f99331a

Bug: 173341412
Fix: 205863279
Test: Build and flash
Change-Id: I042d40623ed34251a212fbf25f72de5647b88835