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 <linus.walleij@linaro.org>
Cc: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
Link: https://lore.kernel.org/r/20211102220203.940290-5-corbet@lwn.net
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Author: Jonathan Corbet