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

This reverts commit c97648b550.

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

include/curl/system.h

@@ -348,8 +348,7 @@
    defined(__ppc__) || defined(__powerpc__) || defined(__arm__) ||      \
    defined(__sparc__) || defined(__mips__) || defined(__sh__) ||        \
    defined(__XTENSA__) ||                                               \
-   (defined(__SIZEOF_LONG__) && __SIZEOF_LONG__ == 4) ||                \
-   (defined(SIZEOF_LONG) && SIZEOF_LONG == 4))
+   (defined(__SIZEOF_LONG__) && __SIZEOF_LONG__ == 4))
 #    define CURL_TYPEOF_CURL_OFF_T     long long
 #    define CURL_FORMAT_CURL_OFF_T     "lld"
 #    define CURL_FORMAT_CURL_OFF_TU    "llu"