MM-53482 : Removed code related to "Enable Preview Features" setting

[EvaGraceSmith]

Summary

1. Removed all remaining references to ServiceSettings.EnablePreviewFeatures on the server and web app
2. Removed the deleteUnusedFeatures cleanup function from the SqlPreferenceStore on the server as well as the
   FeatureTogglePrefix constant used by that code
3. Removed the code from the UserSettingsAdvanced component that used to offer a list of preview features to the user if EnablePreviewFeatures was enabled
4. Removed the code that uses the FeatureTogglePrefix and PRE*RELEASE*FEATURES constants on the web app

Ticket Link

Fixes #23985
Jira https://mattermost.atlassian.net/browse/MM-53482

Screenshots

Release Note

NONE

[mm-cloud-bot]

@EvaGraceSmith: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[mattermost-build]

Hello @EvaGraceSmith,

Thanks for your pull request! A Core Committer will review your pull request soon. For code contributions, you can learn more about the review process here.

[M-ZubairAhmed]

/release-note-none

[mattermost-build]

E2E tests not automatically triggered, because the PR is not in a mergeable state. Please update the branch with the base branch and resolve outstanding conflicts.

[harshilsharma63]

Hi @EvaGraceSmith 👋 Thanks a ton for the PR! Can you please check out the failing CI steps? Mostly due to lint errors I believe.

[harshilsharma63]

Changes look fine overall, but due to unused imports in server-side, the server doesn't even compile. Can you make sure to fix the lint issues and try out the changes?

[harshilsharma63]

Awaiting lint and CI fix. Changes look good otherwise

[marianunez]

Thanks @EvaGraceSmith! LGTM - pending fixing linting errors.

[EvaGraceSmith]

Thank you! I will work on getting the linting errors fixed this weekend!

…

On Thu, Jul 27, 2023 at 5:21 PM Maria A Nunez ***@***.***> wrote: ***@***.**** approved this pull request. Thanks @EvaGraceSmith <https://github.com/EvaGraceSmith>! LGTM - pending fixing linting errors. — Reply to this email directly, view it on GitHub <#24028 (review)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/A3OUH6LXOJCR6ZMUKLY5CDDXSLSWNANCNFSM6AAAAAA2JRHYCI> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[mattermost-build]

This PR has been automatically labelled "stale" because it hasn't had recent activity.
A core team member will check in on the status of the PR to help with questions.
Thank you for your contribution!

[harshilsharma63]

Awaiting server compilation error.

[marianunez]

@EvaGraceSmith just checking in - let us know if there is any help you would need to fix the CI errors