lib: remove useless null assignment

Is it useless to assign null to win32.win32 & 'win32.posix' & posix.win32 & posix.posix since they are be assigned ultimately ?

posix.win32 = win32.win32 = win32; posix.posix = win32.posix = posix;

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines