nir/info: Get rid of uses_interp_var_at_offset

We were using this briefly in the i965 driver to trigger recompiles but we
haven't been using it since we switched to the NIR y-transform lowering
pass.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir.h

@@ -1723,9 +1723,6 @@ typedef struct nir_shader_info {
    /* Whether or not this shader ever uses textureGather() */
    bool uses_texture_gather;
 
-   /** Whether or not this shader uses nir_intrinsic_interp_var_at_offset */
-   bool uses_interp_var_at_offset;
-
    /* Whether or not this shader uses the gl_ClipDistance output */
    bool uses_clip_distance_out;