mtd: rawnand: Remove unused caller_is_module() definition

Commit 260e89a6

 ("mtd: core: tone down suggestion that dev.parent
should be set") removed the only user of caller_is_module() but forgot
to remove the definition itself. Let's remove it now.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>