Remove feature flag yaml for whats new

Following up from a previous MR (listed below), this commit
removes the Feature Flag yaml files for "Whats New".

follow up:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52020

part of:
https://gitlab.com/gitlab-org/gitlab/-/issues/299426#note_490671192

config/feature_flags/development/whats_new_drawer.yml

----
-name: whats_new_drawer
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38975
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/254186
-milestone: '13.3'
-type: development
-group: group::retention
-default_enabled: false

ee/config/feature_flags/development/whats_new_dropdown.yml

----
-name: whats_new_dropdown
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/29183
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/215422
-milestone: '12.10'
-type: development
-group: group::retention
-default_enabled: false