helper/command: drop the TCL variable 'ocd_HOSTOS'

helper/command: drop the TCL variable 'ocd_HOSTOS'

Commit 7a731eb63731 ("Added HostOS variable"), merged in 2009,
adds a TCL global variable 'ocd_HostOS' that reports in a string
the OS of the host.
This was proposed as a workaround for jimtcl that didn't define
the standard TCL variable 'tcl_platform(os)'.

With commit 42f3fb7b7f46 ("Determine platform_tcl() settings with
configure"), merged in 2010 and part of jimtcl 0.70 issued in
early 2011, jimtcl provides the requires TCL standard variable
'tcl_platform(os)'.

The variable 'ocd_HostOS' has never been used by any TCL script
distributed with OpenOCD.

Drop the TCL variable 'ocd_HostOS'.

Change-Id: I27858de35cc9d30df97145ca1ccd24877be4af11
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6189
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>