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/codegen/src/simple_preopt.rs

@@ -1,8 +1,7 @@
 //! A pre-legalization rewriting pass.
 //!
 //! This module provides early-stage optimizations. The optimizations found
-//! should be useful for already well-optimized code. More general purpose
-//! early-stage optimizations can be found in the preopt crate.
+//! should be useful for already well-optimized code.
 
 use crate::cursor::{Cursor, FuncCursor};
 use crate::divconst_magic_numbers::{magic_s32, magic_s64, magic_u32, magic_u64};