firmware: Drop FIRMWARE_IN_KERNEL Kconfig option

It doesn't actually do anything.  Merge its help text into
EXTRA_FIRMWARE.

Fixes: 5620a0d1 ("firmware: delete in-kernel firmware")
Fixes: 0946b2fb ("firmware: cleanup FIRMWARE_IN_KERNEL message")
Signed-off-by: Benjamin Gilbert <benjamin.gilbert@coreos.com>
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>