Delete StlAllocator assignment operator instead of making it private

[lkeegan]

resolves this compiler warning with clang 10:

../inc/Temporary/StlAllocator.h:95:23: error: definition of implicit copy constructor for 'StlAllocator<unsigned char>' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
        StlAllocator& operator=(const StlAllocator&);