MDEV-38383 Fix MDEV-38073 MTR test warning

[ayurchen]

MDEV-38073 MTR test started to fail with a warning after upstream merge from 11.4 a7528a6 because THD responsible for creating SST user became read-only when the server was started with --transaction-read-only=TRUE.
make sure the readonly flag on THDs created for wsp::thd utility class is cleared regardless of the --transaction-read-only value as it is intended only for client-facing THDs.

• The Jira issue number for this PR is: MDEV-______

Description

TODO: fill description here

Release Notes

TODO: What should the release notes say about this change?
Include any changed system variables, status variables or behaviour. Optionally list any https://mariadb.com/kb/ pages that need changing.

How can this PR be tested?

TODO: modify the automated test suite to verify that the PR causes MariaDB to behave as intended.
Consult the documentation on "Writing good test cases".

If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.