Remove windows/qt-installer-windows.qs since no longer used

[ann0see]

Short description of changes
Since Travis Ci is no longer used, the qt-installer-windows.qs file is no longer needed.

CHANGELOG: Internal: Removed outdated qt-installer-windows.qs since we no longer use Travis CI
Context: Fixes an issue?
Fixes: #2515

Does this change need documentation? What needs to be documented and how?
Yes. The COMPILING.md file should be updated.

Status of this Pull Request
Ready for review

What is missing until this pull request can be merged?
Nothing.

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[hoffie]

I usually argue against "can you additionally do $OTHER_THING" in PRs, but in this case I think at least the closely related updating part of COMPILING.md could be done as part of this PR?

I.e. remove the reference to this file and replace it with a reference to the Windows autobuild script?

[ann0see]

@hoffie I did not add the autobuild script but a reference to Qt 5.15.2. I think we should update this file and not direct the user to some files in Git.