Remove ALPHA_INDEX_IME_BUTTON_NAV

Taskbar already stashes when IME is showing, which hides the taskbar
icons in 3 button mode. Hence this extra flag seems redundant and breaks
Transient Taskbar, which allows unstashing taskbar over IME

Test: Install 3P keyboard, swiping taskbar up over IME still shows icons
Test: Connect physical keyboard, icons still hide in 3 button nav when
the IME switcher is showing
Fixes: 264723145

Change-Id: Iafb89e765668d70a1bfad2e136957a2ef8924514