Remove an unused field IMMS#mShortcutInputMethodsAndSubtypes

Since it was originally introduced [1],
IMMS#mShortcutInputMethodsAndSubtypes had been updated only from
IMMS#addShortcutInputMethodAndSubtypes(), which soon became an unused
method [2] then was removed [3].

As a result, IMMS#mShortcutInputMethodsAndSubtypes is now guaranteed
to be always empty.  Hence it can be safely removed.

This is a mechanical code clean up.  There should be no behavior
change.

 [1]: Ibd0f7ef5101013569c303820a3adc9038a97356d
      4e4569da
 [2]: Ib45ca20560c96faa6e2798242f3d1c3fe5156457
      3da92236
 [3]: I3bd814c8b5892b4ef28d6d5622014463df8f1c2b
      59bc6e03

Bug: 120709962
Test: compile
Change-Id: I46becaff97e1e041c6de52eb782c40d34932c113