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/filetests/src/lib.rs

@@ -45,7 +45,6 @@ mod test_interpret;
 mod test_legalizer;
 mod test_licm;
 mod test_optimize;
-mod test_preopt;
 mod test_print_cfg;
 mod test_run;
 mod test_safepoint;
@@ -122,7 +121,6 @@ fn new_subtest(parsed: &TestCommand) -> anyhow::Result<Box<dyn subtest::SubTest>
         "legalizer" => test_legalizer::subtest(parsed),
         "licm" => test_licm::subtest(parsed),
         "optimize" => test_optimize::subtest(parsed),
-        "preopt" => test_preopt::subtest(parsed),
         "print-cfg" => test_print_cfg::subtest(parsed),
         "run" => test_run::subtest(parsed),
         "safepoint" => test_safepoint::subtest(parsed),