i965/vec4: Remove uses_clip_distance from program key.

i965/vec4: Remove uses_clip_distance from program key.

This should never have been in the program key in the first place,
since it's determined by the shader source, not by GL state.  Change
the code to just refer to gl_program::UsesClipDistanceOut directly.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>