ARC: remove #ifdef-ed out include of dead header

There's no (Kconfig) macro CONFIG_BLOCK_DEV_RAM. (CONFIG_BLK_DEV_RAM
does exist though.) But linux/blk.h got killed in 2005 anyway (in a
patch titled "kill blk.h"), so these three lines can be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>