Deprecate InputMethodUtils#isSystemIme() part 3

With my previous two CLs [1][2], InputMethodUtils#isSystemIme() is
used nowhere. Hence we can safely remove this unused method.

 [1]: I9c8518988787b748ebb35fc86fe6beee1d6c633d
      fd70fe8f
 [2]: I33a8489c0597bd881dd8b68b28fc0d99109c11a5
      a3401f2cb0222d058f452885109071e9fd70a6be

Bug: 77730201
Test: compile
Change-Id: I959209c5fef7e9653e09333e860e763f0d4c262c