[Flag cleanup] remove accessibility_customize_text_reading_preview

This bug fix associated with this flag is low risk. The flag has been
soaked in the Trunkfood full for 30 days and tested by 1000+ users.
Based on the flag cleanup policy, we can clean up the flag.

Bug: 307481249
Flag: EXEMPT flag cleanup
Test: Run the Settings app and can still see the text reading preview
Change-Id: I59e03da1719041fb2d17da1d1eed555f31c3d81d