tcl/target, board: remove useless gdb-attach event definitions

Since commit bae76053dc515252dc5c8235b9a848e461080c66
gdb-attach event is defined as halt by default.
Remove useless and in case of bcm281xx wrong definitions of the event.

Change-Id: I8e69780a93722eb9392673303f54d502e71eceb6
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4763
Tested-by: jenkins
Reviewed-by: Steven Stallion <sstallion@gmail.com>