Remove instrgen

[pellared]

It looks like instrgen is no longer maintained. #3682 is the last PR which was submitted more than year a ago.

I propose removing instrgen unless there is someone willing to maintain it.

I believe it is safe to remove the module so since it was never released, and we have the code in the Git history if we wish to resurrect the component in the future.
This change will make CI faster and less flaky.
We would also get rid of this PR: #5494.

CC @pdelewski @MrAlias (as codeowners)

[pdelewski]

@pellared @MrAlias There are few opened PRs that nobody reviewed up to date like #4636, #4058 and so on. To continue work on that we need at least one other person and support from maintainers of this repo.

[ralf0131]

There is an ongoing discussion to donate the golang compile time instrumentation from Alibaba Cloud. We found that the approach shares the similar idea with what InstrGen does and we are working on a solution to combine the two approaches. open-telemetry/community#1961