Remove unused header <linux/sdb.h>

Commit 6a80b30 ("fmc: Delete the FMC subsystem") removed the last user
of <linux/sdb.h>, but left the header file behind.  Nothing uses this file,
delete it now.

Cc: Linus Walleij <[email protected]>
Cc: Alessandro Rubini <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Acked-by: Alessandro Rubini <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Linus Walleij <[email protected]>

Author: Jonathan Corbet