Remove mux field under <sg_link> tag in architecture file.

[soheilshahrouz]

<sg_link> tag has a seg_type field to specify what wire segment type is driven. A <segment> tag specifies which switch type is used to drive the segment via <mux> subtag. I think this makes <mux> subtag under <sg_link> redundant. We should probably remove it.

[AmirhosseinPoolad]

Based on discussion with @soheilshahrouz, we decided to use the mux attribute as an override instead of removing it. Leaving the issue open until the overriding functionality is implemented.