Remove otelhttp/test module

[pellared]

Per open-telemetry/opentelemetry-go#6286 (comment)

When module a has a dependency is used only in tests then it does not come up as an indirect dependency for modules depending on a.

I think this was not true before and that the Go module graph pruning has been improved at some point.

I think that it means that we can get rid of test modules as can use SDK in tests and examples as they do not come up as indirect dependencies in go.mod.

EDIT: The test modules were introduced when we were supporting Go 1.15. https://github.com/open-telemetry/opentelemetry-go-contrib/blob/fe2fb58536fff0f85173f56a9091bca9380cee3c/instrumentation/net/http/otelhttp/test/go.mod
Go module pruning was works for modules requiring at least Go 1.17. See: https://go.dev/ref/mod#graph-pruning and
https://go.dev/doc/go1.17

I plan to do the same for all test modules in separate PRs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.5%. Comparing base (6e7414f) to head (d635839).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6763     +/-   ##
=======================================
- Coverage   75.6%   75.5%   -0.2%     
=======================================
  Files        207     206      -1     
  Lines      19186   19181      -5     
=======================================
- Hits       14512   14484     -28     
- Misses      4239    4259     +20     
- Partials     435     438      +3     

see 4 files with indirect coverage changes