Remove a superfluous assertion, rejected by some warnings.

unsigned is always >= 0.

src/vdp_pesi.c

@@ -2305,7 +2305,6 @@ mpl_init()
 
 	test = MPL_Get(mpl, &sz);
 	AN(test);
-	assert(sz >= 0);
 	assert(sz <= node_alloc_sz);	// no unsigned underflow
 
 	node_alloc_sz += sizeof(struct node);