Modernize the codebase - remove unused imports

[JanVogelsang]

This PR removes all unused imports in C/C++ files.

[JanVogelsang]

Can someone support me with the failing checks? Somehow no errors got reported and all checks pass, but still the pipeline exits with an error code of 1.

[heplesser]

Can someone support me with the failing checks? Somehow no errors got reported and all checks pass, but still the pipeline exits with an error code of 1.

The problem are the compiler warnings:

Make                   | Failed                             |
[2600](https://github.com/nest/nest-simulator/actions/runs/3182211083/jobs/5187939958#step:9:2601)
|                        |                                    |
[2601](https://github.com/nest/nest-simulator/actions/runs/3182211083/jobs/5187939958#step:9:2602)
|                        | Errors  : 0                        |
[2602](https://github.com/nest/nest-simulator/actions/runs/3182211083/jobs/5187939958#step:9:2603)
|                        | Warnings: 4

Known warnings that are to be ignored are defined here

nest-simulator/build_support/parse_build_log.py

Lines 372 to 376 in e1c80d0

	known_warnings = [
	f'{build_dir}/sli/scanner.cc:638:13: warning: this statement may fall through [-Wimplicit-fallthrough=]',
	f'{build_dir}/sli/scanner.cc:668:19: warning: this statement may fall through [-Wimplicit-fallthrough=]',
	f'{build_dir}/sli/scanner.cc:710:13: warning: this statement may fall through [-Wimplicit-fallthrough=]',
	f'{build_dir}/sli/scanner.cc:738:24: warning: this statement may fall through [-Wimplicit-fallthrough=]',

with explicit line numbers. Since you removed one include line from sli/scanner.cc, the line numbers need to be updated.

[jougs]

@JanVogelsang: Can you please fix the conflict? Thanks!

[JanVogelsang]

Well, one of the checks failed because of an unstable connection, as it seems. Is there any way for me to rerun the checks? The tests pass on my branch, so they will also pass here.

[heplesser]

Just one little fix, see inline.

[jougs]

@clinssen, @pnbabu: Can you please check if this PR has consequences for NESTML? And this triggers a question: are includes in the NESTML templates added conditionally depending on what features are actually used in the model or is there just a bunch of default includes?

[jougs]

Have these changes been suggested by a tool? If so, could this be made into a static code check that runs on the CI to reject commits that are adding unused includes? I'd be willing to wait for this PR until such a test is added.

[heplesser]

Have these changes been suggested by a tool? If so, could this be made into a static code check that runs on the CI to reject commits that are adding unused includes? I'd be willing to wait for this PR until such a test is added.

Good point! Obviously, it needs to be a free open source tool.

[JanVogelsang]

I didn't use an open-source tool for this task, however I found one on GitHub, that seems to do the same thing: include what you use

[jougs]

@heplesser: Does your approval mean that we skip the addition of the static test for now? @JanVogelsang: can you please create an issue to remind us of the addition of a such a test?

I'm approving now, as I'm fine with the changes in this PR.

[heplesser]

@jougs Yes, I think it is better to get these changes in place and add the automated check later through a separate issue. I will merge now.