[TASK] SchemaMigrator is stateless

Class SchemaMigrator has an unused property $schema.
Removing this property makes it more obvious
this service is stateless.

Resolves: #101207
Releases: main, 12.4
Change-Id: Icb6fced983129a508152ed79f7511b1b219c23db
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79566


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>