Remove backup implementation of Round for Windows platforms

[cgohlke]

Fixes link error on Visual Studio 2019 14.29.30133 with Windows 10 SDK 10.0.19041.0:

ucrt.lib(api-ms-win-crt-math-l1-1-0.dll) : error LNK2005: round already defined in Convert.obj
   Creating library build\temp.win-amd64-3.10\Release\src\_imaging.cp310-win_amd64.lib and object build\temp.win-amd64-3.10\Release\src\_imaging.cp310-win_amd64.exp
build\lib.win-amd64-3.10\PIL\_imaging.cp310-win_amd64.pyd : fatal error LNK1169: one or more multiply defined symbols found
error: command 'C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Community\\VC\\Tools\\MSVC\\14.29.30133\\bin\\HostX86\\x64\\link.exe' failed with exit code 1169

[radarhere]

This is reverting 9d00686

[cgohlke]

Pillow/src/libImaging/Convert.c

Lines 46 to 51 in b4bf288

	#ifndef round
	double
	round(double x) {
	return floor(x + 0.5);
	}
	#endif

The use of #ifndef round looks wrong here. #ifndef checks whether a macro is not defined but round is a function declared in math.h (or not; it's only part of the C99 standard).

[nulano]

FWIW https://learn.microsoft.com/en-us/cpp/overview/visual-cpp-language-conformance?view=msvc-170#c-standard-library-features-1 suggests that round support was added in VS2015, which is the minimum required for packages compatible with CPython 3.5 or newer.

The oldest supported Visual Studio in winbuild/build_prepare.py is currently VS 2017, and I've been considering increasing that to VS 2019.