target/cortex_m: remove target halted check when removing a breakpoint

target/cortex_m: remove target halted check when removing a breakpoint

Remove the target halted check from cortex_m_remove_breakpoint()
as there is no such check in cortex_m_set_breakpoint() and bp can be
safely removed from the running target.

While on it return the error code from cortex_m_unset_breakpoint()
to higher level.

Change-Id: I2b358c3661feed84297913e9f589bdf1e4de7e64
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4887
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>