Revert "minor #12673 [Form] [Hackday] [2.7] Add a deprecation note about the `form_enctype` helper (mrcmorales)"

This reverts commit e8d98578f4bc5037b8b36968843a7bee1d44641d, reversing
changes made to 0c7df5898f322102a82e54900519197e0e6bdb7a.
Adds deprecation notices for structures to be removed in 3.0.