Clean up unused codegen code

[thomas11]

• Remove unused code from gen/main
• We only need GeneratePackage for Go

This also helps making our code more resilient against upstream p/p changes.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 57.79%. Comparing base (d60a23b) to head (0c2a30c).
Report is 2 commits behind head on master.

Files	Patch %	Lines
provider/cmd/pulumi-gen-azure-native/main.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3538      +/-   ##
==========================================
+ Coverage   57.68%   57.79%   +0.10%     
==========================================
  Files          66       66              
  Lines        8305     8281      -24     
==========================================
- Hits         4791     4786       -5     
+ Misses       3069     3049      -20     
- Partials      445      446       +1     

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[pulumi-bot]

This PR has been shipped in release v2.58.0.