Partially revert "tree-wide: add missing magic checks"

This reverts commit 328540c in the
lib/libvarnishapi/vsc.c file where no magic check could be added to
replace AN(vsm) occurrences.

We should probably look for more potential net loss of assertions from
that commit.

lib/libvarnishapi/vsc.c

@@ -319,6 +319,7 @@ vsc_del_seg(const struct vsc *vsc, struct vsm *vsm, struct vsc_seg **spp)
 	struct vsc_seg *sp;
 
 	CHECK_OBJ_NOTNULL(vsc, VSC_MAGIC);
+	AN(vsm);
 	TAKE_OBJ_NOTNULL(sp, spp, VSC_SEG_MAGIC);
 	AZ(VSM_Unmap(vsm, sp->fantom));
 	if (sp->vj != NULL) {
@@ -342,6 +343,7 @@ vsc_add_seg(const struct vsc *vsc, struct vsm *vsm, const struct vsm_fantom *fp)
 	struct vsc_pt *pp;
 
 	CHECK_OBJ_NOTNULL(vsc, VSC_MAGIC);
+	AN(vsm);
 
 	ALLOC_OBJ(sp, VSC_SEG_MAGIC);
 	AN(sp);