rt2500usb: remove dead link tuning code

Link tuning code from the legacy rt2570 driver turned out to be
harmful and got disabled by the commit d06193f3 ("rt2x00: Disable
link tuning in rt2500usb") in August 2008.

There is no reason to keep this dead code any longer so remove it
(it can still be retrieved from the git history if necessary).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>