[CLEANUP] Drop support for {{#with}} keyword

[chancancode]

This was dropped on Ember side a while back, just never cleaned up

Ironically this probably meant when Ember dropped the deprecation/ error, nothing was actually stopping {{#with}} from anymore, so perhaps for a while we have actually re-introduced support for the syntax for some time?

Extracted from #1557

[chancancode]

Ironically this probably meant when Ember dropped the deprecation/ error, nothing was actually stopping {{#with}} from anymore, so perhaps for a while we have actually re-introduced support for the syntax for some time?

Yeah,I think we actually did, just tried in on 5.4 and {{#with}} certainly works there.

Specifically, in here: emberjs/ember.js#19632 / emberjs/ember.js#19638

This is partially why, among other reasons, when removing a deprecated feature, we typically want to swap the deprecate call for an assert call for at least 1-2 LTS versions.