cmd/compile/internal/importer,go/internal/gcimporter: remove iimport.go export data importers

[timothy-king]

Tracking bug for removing the iimport.go files for the iexport data format in the compiler.

The unified goexperiment was removed in 1.21. Once #64751 happens, the bootstrap compiler will be above 1.21, I am not aware of a reason these importers could be use in the compiler.

x/tools will still have a copy for gopls, which creates this format, and support for building tools that support export data created by older compilers.

[gabyhelp]

Related Issues and Documentation

• go/importer: remove support for (now defunct) binary export format (reminder issue) #27999 (closed)
• x/tools/internal/gcimporter: update iexport.go to support generics #48392 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[timothy-king]

Closing this issue. I am going to think through the implications of removing go/internal/gcimporter/iimport.go support a bit more. I'll open a new issue when I have reached a conclusion.