target: remove unimplemented soft_reset_halt implementations

Let the default handler issue an unsupported warning rather than using
empty handler routines that may/may not issue a unsupported warning.

Change-Id: Iafe3e45146981a4cfae39771c3ab7370ac86da48
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1535


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Hsiangkai Wang <hsiangkai@gmail.com>