Bug 1413475 - Remove ssize_t definition from mozjemalloc.cpp. r=njn

The only use of ssize_t in mozjemalloc was removed in bug 1403444.

--HG--
extra : rebase_source : 98dde18a08a9a64b2b61698c43cbf3fc7eb74b5d

memory/build/mozjemalloc.cpp

@@ -182,12 +182,6 @@ getenv(const char* name)
   return nullptr;
 }
 
-#if defined(_WIN64)
-typedef long long ssize_t;
-#else
-typedef long ssize_t;
-#endif
-
 #define MALLOC_DECOMMIT
 #endif