Core: Replace header guards with #pragma once, remove unused includes

[sruon]

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• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Time to clean-up the C++ code and make it more consistent.

Starting simple:

• Convert to #pragma once
• Drop unused headers

Next steps will include:

• Reducing the headers we pull in, in a few places
• Introducing std::ranges/std::erase_if where possible
• Replacing ternaries
• Getting rid of C casts
• Standardizing on dynamic_cast for type checks
• Replacing enums with scoped enums
• @zach2good will probably add to the list

Steps to test these changes

[zach2good]

Excellent