[gapic-generator] Remove version check for `grpcio` from pqc showcase integration tests

[ohmayr]

Once the lower bound for grpcio is bumped to >= 1.83.0 across our client libraries, remove the version check from the pqc showcase integration tests in gapic-generator.

Also remove showcase_pqc nox session from gapic-generator since the tests will run successfully by default and no longer be skipped.