helper/startup.tcl: remove proc exit

helper/startup.tcl: remove proc exit

The TCL command exit is already replaced by the OpenOCD command
exit in server/telnet_server.c, no need to redefine it in the
script.
Moreover, the implementation is broken because the proc ocd_throw
has been removed in mid 2008 with pre-git-era commit dfbb9f3e89ae
in svn rev 849.

Remove completely the unused proc exit.

Change-Id: I0365d740eccc47631eb459aab77b865b0877c1f7
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4986
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>