Fix a dead code warning from the new Rust compiler.

On ISAs with no instruction predicates, just emit an unimplemented!()
stub for the check_instp() function. It is unlikely that a finished ISA
will not have any instruction predicates.

lib/cretonne/src/isa/intel/enc_tables.rs

@@ -1,7 +1,6 @@
 //! Encoding tables for Intel ISAs.
 
 use ir::InstructionData;
-use ir::instructions::InstructionFormat;
 use ir::types;
 use isa::enc_tables::{Level1Entry, Level2Entry};
 use isa::constraints::*;