i965/gs: Remove the dependency on the VS VUE map.

i965/gs: Remove the dependency on the VS VUE map.

Because we only support geometry shaders in core profile, we can safely
ignore any driver-extending of VS outputs.

Those are:
- Legacy userclipping (doesn't exist in core profile)
- Edgeflag copying (Gen4-5 only, no GS support)
- Point coord replacement (Gen4-5 only, no GS support)
- front/back color hacks (Gen4-5 only, no GS support)

v2: Rebase; leave a comment about why SSO works.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>