Cranelift: Remove unused ABICaller::signature method (#4621)

And the `ABICallerImpl::ir_sig` field that was used to implement that
method. This removes 56 bytes from the size of `ABICallerImpl` and gives us
speed ups to compilation of about 7% on all benchmarks.

```
compilation :: nanoseconds :: benchmarks/pulldown-cmark/benchmark.wasm

  Δ = 8205119.48 ± 4069474.25 (confidence = 99%)

  main.so is 0.91x to 0.97x faster than feature.so!
  feature.so is 1.03x to 1.10x faster than main.so!

  [117729152 132258110.36 167484097] main.so
  [107486500 124052990.88 138008797] feature.so

compilation :: nanoseconds :: benchmarks/bz2/benchmark.wasm

  Δ = 4645258.32 ± 1981104.59 (confidence = 99%)

  main.so is 0.92x to 0.97x faster than feature.so!
  feature.so is 1.03x to 1.08x faster than main.so!

  [76562171 85504479.28 93116863] main.so
  [75180650 80859220.96 90591978] feature.so

compilation :: nanoseconds :: benchmarks/spidermonkey/benchmark.wasm

  Δ = 150575617.54 ± 65021102.57 (confidence = 99%)

  main.so is 0.92x to 0.97x faster than feature.so!
  feature.so is 1.03x to 1.08x faster than main.so!

  [2573089039 2843117485.10 3175982602] main.so
  [2559784932 2692541867.56 3143529008] feature.so
```

cranelift/codegen/src/machinst/abi.rs

@@ -191,9 +191,6 @@ pub trait ABICaller {
     /// Get the number of arguments expected.
     fn num_args(&self) -> usize;
 
-    /// Access the (possibly legalized) signature.
-    fn signature(&self) -> &Signature;
-
     /// Emit a copy of an argument value from a source register, prior to the call.
     /// For large arguments with associated stack buffer, this may load the address
     /// of the buffer into the argument register, if required by the ABI.