remove mysql relation to drop MariaDB support

[DnPlas]

Enhancement Description

CKF 1.8 is dropping support for MariaDB, which means the charm has to drop all the code that handles this relation.
The support has to be dropped from the kfp-api charm.

Suggested changes:

• Remove the relation
• Stop observing and remove event handlers from the charm code as well, like this

To Reproduce

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Environment

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Relevant log output

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Additional context

No response

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-6668.

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