Remove MySqlSchemaBehavior in favor of a warning

[lauxjpn]

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We should consider removing MySqlSchemaBehavior in favor of a warning that throws by default if a schema is being used, but can be ignored if explicitly configured so.

Instead of a SchemaBehavior() call, users could then use a SchemaNameTranslator() call to directly specify a custom schema name translator. The default one would just ignore the schema name and just use the object name. It would be used in case the warning gets ignored and no custom schema name translator has been set. It also makes sense to implicitly ignore the warning if a custom schema name translator has been set

This would be more in line with general EF Core practices.