target/cortex_m: simplify cortex_m_unset_breakpoint()

target/cortex_m: simplify cortex_m_unset_breakpoint()

Cortex-M uses only 2 byte breakpoint instruction.
cortex_m_unset_breakpoint() does not need to check breakpoint->length,
use the length as the size argument in target_write_memory()

Change-Id: I20bb869f6abce2fc61f0469e34a638bc4dc6f7ce
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4889
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Jānis Skujenieks <janis.skujenieks@gmail.com>
Reviewed-by: Christopher Head <chead@zaber.com>