mtd: mtdcore: cleanup mtdcore.h a little

Remove useless extern qualifiers. Not that this is a problem, but we more often
declare function prototypes without 'extern', so this is just about being more
consistent. And I am going to add a couple more prototypes here.

Additionally, remove a useless comment.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>