Revert "curl/system.h: fix compilation with gcc on AIX PPC and IA64 HP-UX"

This reverts commit c97648b5.

SIZEOF_LONG should not be checked in system.h since that macro is only
defined when building libcurl.

Ref: https://github.com/curl/curl/pull/2186#issuecomment-354767080
Ref: https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html