Fix - DataMigration assigns a single discriminator value to multiple model types

[hitenjava]

This is to resolve - #2728

The model hierarchy looks like this right now:

> TaskOutput.MigrateSqlServerSqlDbTaskOutput.MigrateSqlServerSqlDbTaskOutputError: ErrorOutput
> TaskOutput.MigrateSqlServerSqlMITaskOutput.MigrateSqlServerSqlMITaskOutputError: ErrorOutput

For a given discriminator name (such as TaskOutput.MigrateSqlServerSqlDbTaskOutput.MigrateSqlServerSqlDbTaskOutputError), all of the possible values (such as ErrorOutput) must be distinct.
Please change the discriminator values to DbErrorOutput and MIErrorOutput, or whatever values you prefer, as long as they are distinct.

Instead of changing the discriminator, removing the common base class so that the discriminator values do not apply for the same object.

PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• Except for special cases involving multiple contributors, the PR is started from a fork of the main repository, not a branch.
• If applicable, the PR references the bug/issue that it fixes.
• Swagger files are correctly named (e.g. the api-version in the path should match the api-version in the spec).

Quality of Swagger

• I have read the contribution guidelines.
• My spec meets the review criteria:
  • The spec conforms to the Swagger 2.0 specification.
  • The spec follows the guidelines described in the Swagger checklist document.
  • Validation tools were run on swagger spec(s) and have all been fixed in this PR.

[AutorestCI]

Automation for azure-libraries-for-java

Nothing to generate for azure-libraries-for-java

[AutorestCI]

Automation for azure-sdk-for-node

Nothing to generate for azure-sdk-for-node

[AutorestCI]

Automation for azure-sdk-for-python

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-python#2071

[AutorestCI]

Automation for azure-sdk-for-go

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-go#1457

[olydis]

LGTM