Remove redundant sizeof "size_t" check

[weitjong]

The result variables: HAVE_${VARIABLE}, ${VARIABLE}, ${VARIABLE}_CODE, etc. do not seem to be referenced anywhere in the CMake build script. Unless I was mistaken on how the check is being used.

The motivation for me to have the redundant check removed is because otherwise it would cause an error in my SDL initial build tree generation with my own custom Emscripten.cmake toolchain file. My custom CMake toolchain file is much simpler version than the official one provided by EMSDK, which does not provide the "hack" version of the CheckTypeSize.cmake.

But in any case, there should be no harm to have this redundant check removed for other use cases.

My Web CI build can be found here: https://github.com/alpha-scorpii-b/SDL/runs/3383479743?check_suite_focus=true

[madebr]

SIZEOF_SIZE_T is indeed never used.
It might however be useful to detect CHERI systems, where sizeof(size_t) != sizeof(void*). (I don't know SDL supports any such platform)
See this fmtlib pr for more info.

[icculus]

This is fine; if we find ourselves on a system that needs it, we'll deal with it.