Remove dead code from MetricsContainer

[bjchambers]

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• [*] Make sure the PR title is formatted like:
  [BEAM-<Jira issue #>] Description of pull request
• [*] Make sure tests pass via mvn clean verify. (Even better, enable
  Travis-CI on your fork and ensure the whole test matrix passes).
• [*] Replace <Jira issue #> in the title with the actual Jira issue
  number, if there is one.
• [*] If this contribution is large, please file an Apache
  Individual Contributor License Agreement.

---

[bjchambers]

R: @tgroh
CC: @pabloem

[tgroh]

LGTM pending green

[bjchambers]

Jenkins failure seems unrelated: https://builds.apache.org/job/beam_PreCommit_MavenVerify/org.apache.beam$beam-runners-apex/5034/testReport/junit/org.apache.beam.runners.apex.translation/ParDoBoundTranslatorTest/testAssertionFailure/