target: Remove "-variant" argument

Remove this underutilized feature. Despite the fact that a lot of configs
specifies a arbitrary "variant", only the xscale target actually defines
any.

In the case of xscale, the use of -variant is dubious since

1) it's used as a redundant irlen specifier,
2) it carries a comment that it doesn't really need it and
3) only two xscale configs even specify it.

If there's a future target that needs a variant set, a target specific
option could be added when needed.

Change-Id: I1ba25a946f0d80872cbd96ddcc48f92695c4ae20
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2283
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>