[5.8] Remove unused Migrator $notes dynamic property

[antonkomarev]

Migrator class has dynamic $this->notes property, which not using.
It looks like legacy code because there is note method, but it uses $this->output to write notes.

protected function note($message) { if ($this->output) { $this->output->writeln($message); } }

[staudenmeir]

#24811 forgot to remove the property.

[antonkomarev]

@staudenmeir, thanks for the link!