Remove MYSQL_FLAVOR from all Docker images

[frouioui]

Description

This is a follow-up of the work done in #13135. We can now remove the MYSQL_FLAVOR environment variable from all of our docker images.

Related Issue

• Part of Upgrade the mysqld version and support major version upgrades in vitess-operator #14160

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

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Backward compatibility

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