fix($compile): remove the preAssignBindingsEnabled flag

Closes #15782

BREAKING CHANGE:

Previously, the `$compileProvider.preAssignBindingsEnabled` flag was supported.
The flag controlled whether bindings were available inside the controller
constructor or only in the `$onInit` hook. The bindings are now no longer
available in the constructor.

To migrate your code:

1. If you haven't invoked `$compileProvider.preAssignBindingsEnabled()` you
don't have to do anything to migrate.

2. If you specified `$compileProvider.preAssignBindingsEnabled(false)`, you
can remove that statement - since AngularJS 1.6.0 this is the default so your
app should still work even in AngularJS 1.6 after such removal. Afterwards,
migrating to AngularJS 1.7.0 shouldn't require any further action.

3. If you specified `$compileProvider.preAssignBindingsEnabled(true)` you need
to first migrate your code so that the flag can be flipped to `false`. The
instructions on how to do that are available in the "Migrating from 1.5 to 1.6"
guide:
https://docs.angularjs.org/guide/migration#migrating-from-1-5-to-1-6
Afterwards, remove the `$compileProvider.preAssignBindingsEnabled(true)`
statement.