target: fix broken Cortex-R4 support

This regression was caused due to the recent addition of R4 support and
the removal of the bulk_write_memory handler.

Change-Id: Ide692737f235c0e9906becb6f3502ba52c5907aa
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1246
Tested-by: jenkins

src/target/cortex_a.c

@@ -2856,7 +2856,6 @@ struct target_type cortexr4_target = {
 
 	.read_memory = cortex_a8_read_memory,
 	.write_memory = cortex_a8_write_memory,
-	.bulk_write_memory = cortex_a8_bulk_write_memory,
 
 	.checksum_memory = arm_checksum_memory,
 	.blank_check_memory = arm_blank_check_memory,