Revert "[analyzer/cfe] Parse final class modifier."

This reverts commit 2f1372b4603c9e9b7e20cdf3adf2336ee3da4385.

Reason for revert: Breaks the build. Unsure if there are other affected failures.

Original change's description:
> [analyzer/cfe] Parse final class modifier.
>
> Change-Id: Ia554c4f8f9617cc883a472eeb819aab2fd849077
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/278090
> Reviewed-by: Johnni Winther <johnniwinther@google.com>
> Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
> Commit-Queue: Kallen Tu <kallentu@google.com>

TBR=scheglov@google.com,johnniwinther@google.com,kallentu@google.com,dart-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: I5dd74202bd46d69b7261d6d30d8cf3b1a40dbc99
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279040
Reviewed-by: Jake Macdonald <jakemac@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>