Revert "does the umem backend affect the amount of malloc NULL returns in vtest?"

This reverts commit 8ea006ee.

does not seem to make a difference, trying to narrow down using other
means (different platforms)

bin/varnishd/storage/storage_umem.c

@@ -94,7 +94,7 @@ static umem_cache_destroy_f umem_cache_destroyf = NULL;
 static umem_cache_alloc_f umem_cache_allocf = NULL;
 static umem_cache_free_f umem_cache_freef = NULL;
 
-static const char * const def_umem_options = "perthread_cache=0";
+static const char * const def_umem_options = "perthread_cache=0,backend=mmap";
 static const char * const env_umem_options = "UMEM_OPTIONS";
 
 /* init required per cache get: