Delete keyboard_category_enabled flag (1/2)

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 declation & usage since no longer
need this flag.

Flag: EXEMPT flag removal
Bug: 332661766
Test: build
Test: atest HapticFeedbackVibrationProviderTest VibrationSettingsTest
Change-Id: Id0dde9986b954f08c31bb0ed652a47eb815575f2