rtl_arena_alloc must never obey AMode_SYSTEM

...as e.g. the read+write+exec arena used in
bridges/source/cpp_uno/shared/vtablefactory.cxx has specific requirements on
obtained memory.  This broke recent 8b9968a2
"fdo#72755: Only use double mmap as fallback" for e.g. --enable-ooenv (which
sets G_SLICE).  This is a partial revert of
ce906b80 "skip tricky allocators on
G_SLICE=always-malloc."

Change-Id: I5a5fb7c3c920a7856381e6c55638137c625b5111

sal/rtl/alloc_arena.cxx

@@ -28,8 +28,6 @@
 #include <string.h>
 #include <stdio.h>
 
-extern AllocMode alloc_mode;
-
 /* ================================================================= *
  *
  * arena internals.
@@ -950,9 +948,6 @@ SAL_CALL rtl_arena_alloc (
     {
         sal_Size size;
 
-        if (alloc_mode == AMode_SYSTEM)
-            return rtl_allocateMemory(*pSize);
-
         size = RTL_MEMORY_ALIGN((*pSize), arena->m_quantum);
         if (size > arena->m_qcache_max)
         {
@@ -1022,12 +1017,6 @@ SAL_CALL rtl_arena_free (
 {
     if (arena != 0)
     {
-        if (alloc_mode == AMode_SYSTEM)
-        {
-            rtl_freeMemory(addr);
-            return;
-        }
-
         size = RTL_MEMORY_ALIGN(size, arena->m_quantum);
         if (size > arena->m_qcache_max)
         {