intel/vec4: Delete vec4_visitor::emit_lrp

Effectivley unused since dd7135d55d ("intel/compiler: Use the flrp
lowering pass for all stages on Gen4 and Gen5").  I had intended to
remove this code as part of that series, but I forgot.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/intel/compiler/brw_vec4.h

@@ -236,9 +236,6 @@ public:
    vec4_instruction *emit_minmax(enum brw_conditional_mod conditionalmod, dst_reg dst,
                                  src_reg src0, src_reg src1);
 
-   vec4_instruction *emit_lrp(const dst_reg &dst, const src_reg &x,
-                              const src_reg &y, const src_reg &a);
-
    /**
     * Copy any live channel from \p src to the first channel of the
     * result.