Delete old cranelift-preopt crate (#5642)

Most of these optimizations are in the egraph `cprop.isle` rules now,
making a separate crate unnecessary.

Also I think the `udiv` optimizations here are straight-up wrong (doing
signed instead of unsigned division, and panicking instead of preserving
traps on division by zero) so I'm guessing this crate isn't seriously
used anywhere.

At the least, bjorn3 confirms that cg_clif doesn't use this, and I've
verified that Wasmtime doesn't either.

Closes #1090.

cranelift/docs/testing.md

@@ -262,7 +262,7 @@ Test the instruction shrinking pass.
 The shrink pass is run on each function, and then results are run
 through filecheck.
 
-### `test preopt`
+### `test simple_preopt`
 
 Test the preopt pass.