Delete keyboard_category_enabled flag (2/2)

As keyboard_category_enabled feature has launched, remove the feature
flag since it's unnessary.

And, this CL is the preparation of removing CATEGORY_KEYBOARD vibration
attribute and logic, remove the flag usage ing settings since no longer
need this flag.

Flag: EXEMPT flag removal
Bug: 332661766
Test: build
Test: atest KeyboardVibrationTogglePreferenceControllerTest
Change-Id: I19cecb977d52a74b26eea1f494052e0e852359c4