Remove rayon dependency of cranelift-isle (#5101)

Using rayon adds a lot of dependencies to Cranelift. The total
unparallelized time the code that uses rayon takes is less than half a
second and it runs at compile time, so there is pretty much no benefit
to parallelizing it.

cranelift/isle/isle/Cargo.toml

@@ -11,7 +11,6 @@ version = "0.90.0"
 [dependencies]
 log = { workspace = true, optional = true }
 miette = { version = "5.1.0", optional = true }
-rayon = "^1.5"
 
 [dev-dependencies]
 tempfile = "3"