[BEAM-1869] Remove GetProducingTransformInternal

[tgroh]

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This isn't part of the PValue API, so remove from POutputValueBase.

[tgroh]

R: @kennknowles

[kennknowles]

LGTM if LGTJenkins

[coveralls]

Coverage increased (+0.006%) to 70.139% when pulling 3d53b72 on tgroh:remove_get_producing_transform_internal into 844e53e on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/9109/
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[coveralls]

Coverage decreased (-0.0006%) to 70.132% when pulling 3d53b72 on tgroh:remove_get_producing_transform_internal into 844e53e on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/9110/
--none--