Trim a few comments on platforms that did not implement mmap of /dev/kmem.

After r307332, no platforms implement mmap for /dev/kmem, so the lack of
it for these platforms is no longer unique.

sys/mips/mips/mem.c

@@ -151,12 +151,6 @@ int
 memmmap(struct cdev *dev, vm_ooffset_t offset, vm_paddr_t *paddr,
     int prot, vm_memattr_t *memattr)
 {
-	/*
-	 * /dev/mem is the only one that makes sense through this
-	 * interface.  For /dev/kmem any physaddr we return here
-	 * could be transient and hence incorrect or invalid at
-	 * a later time.
-	 */
 	if (dev2unit(dev) != CDEV_MINOR_MEM)
 		return (-1);
 

sys/sparc64/sparc64/mem.c

@@ -43,7 +43,7 @@ __FBSDID("$FreeBSD$");
 /*
  * Memory special file
  *
- * NOTE: other architectures support mmap()'ing the mem and kmem devices; this
+ * NOTE: other architectures support mmap()'ing the mem device; this
  * might cause illegal aliases to be created for the locked kernel page(s), so
  * it is not implemented.
  */