Remove suppression for PHPStan errors fixing which in 2.x would be a BC break

|    Q        |   A
|------------ | ------
| BC Break    | no
| Version     | 3.0

Something along these lines:
```sh
$ grep "BC break" phpstan.neon.dist 
        # removing it would be BC break
        # declaring $tableName in AbstractSchemaManager::_getPortableTableIndexesList() non-optional will be a BC break
        # changing these would be a BC break, to be done in next major
```