Remove (action) and {{action}}

[NullVoxPopuli]

Blocked by:

• Implement RFC#1006, Deprecate (action) and {{action}}  #20653
• and v6 (this is a breaking change, so we can't merge this during 5.x)

Changes

• remove (action) helper
• remove {{action}} modifier
• update tests that use either of the above to use either fn or on
• delete tests that no longer make sense