drivers/block: remove redundant 'default n' from Kconfig-s

'default n' is the default value for any bool or tristate Kconfig
setting so there is no need to write it explicitly.

Also since commit f467c564 ("kconfig: only write '# CONFIG_FOO
is not set' for visible symbols") the Kconfig behavior is the same
regardless of 'default n' being present or not:

    ...
    One side effect of (and the main motivation for) this change is making
    the following two definitions behave exactly the same:

        config FOO
                bool

        config FOO
                bool
                default n

    With this change, neither of these will generate a
    '# CONFIG_FOO is not set' line (assuming FOO isn't selected/implied).
    That might make it clearer to people that a bare 'default n' is
    redundant.
    ...
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/Kconfig

@@ -461,7 +461,6 @@ config BLK_DEV_RBD
 	select LIBCRC32C
 	select CRYPTO_AES
 	select CRYPTO
-	default n
 	help
 	  Say Y here if you want include the Rados block device, which stripes
 	  a block device over objects stored in the Ceph distributed object

drivers/block/drbd/Kconfig

@@ -11,7 +11,6 @@ config BLK_DEV_DRBD
 	depends on PROC_FS && INET
 	select LRU_CACHE
 	select LIBCRC32C
-	default n
 	help
 
 	  NOTE: In order to authenticate connections you have to select

drivers/block/zram/Kconfig

@@ -3,7 +3,6 @@ config ZRAM
 	tristate "Compressed RAM block device support"
 	depends on BLOCK && SYSFS && ZSMALLOC && CRYPTO
 	select CRYPTO_LZO
-	default n
 	help
 	  Creates virtual block devices called /dev/zramX (X = 0, 1, ...).
 	  Pages written to these disks are compressed and stored in memory
@@ -18,7 +17,6 @@ config ZRAM
 config ZRAM_WRITEBACK
        bool "Write back incompressible page to backing device"
        depends on ZRAM
-       default n
        help
 	 With incompressible page, there is no memory saving to keep it
 	 in memory. Instead, write it out to backing device.