Tpetra: Fix #3095 (remove deprecated *Platform)

Created by: mhoemmen

@trilinos/tpetra @trilinos/stratimikos

Description

Remove the long-deprecated Tpetra::DefaultPlatform, Tpetra::MpiPlatform, and Tpetra::SerialPlatform classes. Purge some Stratimikos examples (that haven't built for years anyway) that use them.

Note: This change does not affect the Xpetra classes of the same names.

Motivation and Context

Purge deprecated functionality before the next Trilinos release.

Related Issues

• Closes #3095 (closed)