Remove backwards compatible glue for older buildpack APIs

[natalieparellano]

Summary

#1148 removes support for buildpack APIs < 0.7. However, it leaves intact backwards compatible "glue" we added to help 0.2 buildpacks work with 0.3+ buildpacks. We should remove this glue now that 0.2 is no longer supported.

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Proposal

Talked about this in Working Group - newer buildpacks may have been using the glue without realizing the behavior is deprecated. It could be surprising if things that used to work start breaking even though the API hasn't changed. We decided that we should remove the glue anyway, and document what error message would surface if buildpacks are still relying on it.

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Related

RFC 0110