refactor: Clean up mysqli transactions

[kamil-tekiela]

Description
Turning off autocommit and starting a transaction makes very little sense. Unless you expect users to issue manual commit statements, the begin transaction is sufficient. Comparing it to the other Connection classes, only the mysqli one does that, which can be considered a bug. If you would like to allow the user to issue manual commit statements then keep only autocommit and remove begin and commit. Additionally, these functions always return true (because of exception mode) and so the if statements and return statements are not meaningful.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[mergeable]

Hi there, kamil-tekiela! 👋

Thank you for sending this PR!

We expect the following in all Pull Requests (PRs).

• PRs must be sent to the appropriate branch
• All git commits must be GPG-signed
• Must follow our style guide
• Be commented in the PHP source file
• Be documented in the user guide
• Be unit tested
• Pass all checks in GitHub Actions

Important

We expect all code changes or bug-fixes to be accompanied by one or more tests added to our test suite to prove the code works.

If pull requests do not comply with the above, they will likely be closed. Since we are a team of volunteers, we don't have any more time to work
on the framework than you do. Please make it as painless for your contributions to be included as possible.

See https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md

Sincerely, the mergeable bot 🤖

[michalsn]

I had to read up on it, and it looks like you're right. Thanks.