cmake: delete redundant macro definition `SECURITY_WIN32`
Stop explicitly defining `SECURITY_WIN32` in CMake builds.
No other build systems define this macro, because it's unconditionally
defined in `lib/curl_sspi.h` already. This is the only curl source using
the `sspi.h` and `security.h` Win32 headers, and no other Win32 headers
need this macro.
Reviewed-by: Jay Satiro
Closes #10341