[MinGW] Remove unused WinAPI includes/defines from RVO2.

[bruvzg]

Remove unused and conflicting WinAPI defines to fix MinGW cross-build, and unused RVO.h header (part of RVOSimulator).

Fixes #34776 (comment).
Fixes #36104.

[Calinou]

If relevant, could this be sent upstream as well if it's still active?

[AndreaCatania]

Are you sure that this doesn't lead to incorrect RVO behaviour on Windows?

[bruvzg]

If relevant, could this be sent upstream as well if it's still active?

It's last update in 2016.

Are you sure that this doesn't lead to incorrect RVO behaviour on Windows?

I have tested demo project form the #34776 top comment on macOS and Windows and didn't notice any difference.

SDKDDKVer.h - used to define _WIN32_WINNT, that's already defined by Godot build config.
NOMINMAX - disables deprecated macros, already defined by Godot build config.
windows.h with significant portion of it disabled by WIN32_LEAN_AND_MEAN and NOxxx - not sure why it's there.
__declspec()s - only used if RVO is built as Windows .dll.
_USE_MATH_DEFINES - used to define M_ constants, but non of these are used in the RVO.

[akien-mga]

Thanks!