iwlagn: remove iwlagn_build_addsta_hcmd

This function is not needed:
 * we already have the "cmd" input to it in the
   same type (and on the stack elsewhere)
 * the "legacy_reserved" parameter is never set,
   so will always be zero

Remove the function and the stack copy of the
input command.

This is still left from when iwlegacy was part
of the driver -- then we needed a translation
for the command for 3945.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>