[3.0 Cleanup] Removal of the SecurityContext 

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

As follow up for PR https://github.com/symfony/symfony/pull/11690 in 2.6, I have removed the `SecurityContext`, `SecurityContextInterface` and `security.context` service. I have split the parts in separate commits to make it easier to review but I can squash them if desired.

- [ ] Submit changes to the documentation

I will submit the changes to the documentation when a branch is available to do so.
changing the array syntax should not be done yet. We have a separate discussion about this