Remove contact form legacy admin files (#46255)

* remove now unused files

* do not initialize Admin, it is gone

* changelog

* update phan baseline

* remove obsolete tests

* update phan baseline

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/20243251988

Upstream-Ref: Automattic/jetpack@00bd872

Author: CGastrell

CHANGELOG.md

@@ -25,6 +25,7 @@ This is an alpha version! The changes listed here are not final.
 - Update package dependencies.
 
 ### Removed
+- Forms: remove admin class files
 - Forms: Removes classic Admin initialization code
 
 ### Fixed