Removed deprecated build-in encoder

[alexvanboxel]

Component(s)

receiver/googlecloudpubsub

Is your feature request related to a problem? Please describe.

As the receiver supports encoding extensions, and the build-in encoders have alternatives. Remove the build-in encoders to simplify the code and remove dependencies.

The removal was announced in the README for 1.132.0.

Describe the solution you'd like

Remove build-in encoders.

Describe alternatives you've considered

No response

Additional context

I am the code owner, and the removal has been announced for 0.132.0

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[github-actions]

Pinging code owners:

• receiver/googlecloudpubsub: @alexvanboxel

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