Remove TODO item in IMMS#canShowInputMethodPickerLocked

As CL[1] has already supported to show IME picker on the
external/Virtual display with the self reported displayId of the
current focused client, so currently canShowInputMethodPickerLocked
for Single-Session IME usage with switching focus displays
should be enough to check if the given client is the focused client.

Remove the TODO item to prevent the misleading of not support
multi-display for IME picker.

[1]: Ic7d7c5a7ad8005a3fbd9d1c1b73e3c5a39a07001

Bug: 238972096
Test: atest InputMethodPickerTest
Change-Id: I2bdf3c2a8325d95025196782b9fdaae7441124b7