Remove migration table from generated comparison schema

[wmouwen]

Q	A
Type	bug
BC Break	no
Fixed issues	#1406

Summary

doctrine/orm#11374

Schemas generated from metadata cause the event ToolEvents::postGenerateSchema to be triggered in the method SchemaTool::getSchemaFromMetadata. It would be helpful to do the same for schemas generated for comparison in SchemaTool::createSchemaForComparison. This PR introduces the new event ToolEvents::postGenerateComparisonSchema.

This new event, given that the PR in doctrine/orm proceeds, can be utilized in a listener to remove the metadata table from the comparison schema.

PR relates to an attempt to fix #1406. It requires doctrine/orm#11374 and is a requirement of doctrine/DoctrineMigrationsBundle#529.