MHV-59994 remove SM phase 1 feature flag

Note: Delete the description statements, complete each step. None are optional, but can be justified as to why they cannot be completed as written. Provide known gaps to testing that may raise the risk of merging to production.

Summary

Feature flag cleanup: mhv_secure_messaging_to_phase_1

Related issue(s)

Testing done

[ ] New code is covered by unit tests
Describe what the old behavior was prior to the change
Describe the steps required to verify your changes are working as expected. Exclusively stating 'Specs run' is NOT acceptable as appropriate testing
If this work is behind a flipper:
- Tests need to be written for both the flipper on and flipper off scenarios. Docs.
- What is the testing plan for rolling out the feature?

Screenshots

Note: Optional

What areas of the site does it impact?

(Describe what parts of the site are impacted andifcode touched other areas)

Acceptance criteria

[ ] I fixed|updated|added unit tests and integration tests for each feature (if applicable).
[ ] No error nor warning in the console.
[ ] Events are being sent to the appropriate logging solution
[ ] Documentation has been updated (link to documentation)
[ ] No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
[ ] Feature/bug has a monitor built into Datadog (if applicable)
[ ] If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
[ ] I added a screenshot of the developed feature

Requested Feedback

(OPTIONAL)What should the reviewers know in addition to the above. Is there anything specific you wish the reviewer to assist with. Do you have any concerns with this PR, why?

department-of-veterans-affairs / vets-api