i965/vec4: remove the generator hack for dual instanced GS

i965/vec4: remove the generator hack for dual instanced GS

This hack was introduced in commit 03ac2c7223f7645e3:
i965/gs: Fix up gl_PointSize input swizzling for DUAL_INSTANCED gs

Specifically to fixup the code we emitted to deal with gl_PointSize inputs
in dual instance mode, where we were emitting a MOV to copy the point
size from .w (where the hardware delivers it) to .x (because code will
expect this to be a float). This meant that we were emitting a MOV
to an ATTR destination that could have a width of 4 (in dual instanced
mode) so it was necessary to fix the execution size and regioning of the
instruction.

Fortunately, Ken fixed this in 67c5d00273ca2:
i965/vec4/gs: Stop munging the ATTR containing gl_PointSize.

by using a WWWW swizzle instead of a MOV, and as the commit log in that
patch states, we no longer emit instructions with ATTR destinations, so
that makes the fixup code in the generator unnecessary.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>