Turn off memfd by default, at least for this upcoming release. (#3774)

Since memfd support just landed, and has had only ~0.5 weeks to bake
with fuzzing, we want to make release 0.34.0 of Wasmtime without it
enabled by default. This PR disables memfd by default; it can be enabled
by specifying the `memfd` feature for the `wasmtime` crate, or when
building the commandline binary.

We plan to explicitly add memfd-enabled fuzzing targets, let that go for
a while, then probably re-enable memfd in the subsequent release if no
issues come up.

Cargo.toml

@@ -94,7 +94,6 @@ default = [
   "wasmtime/wat",
   "wasmtime/parallel-compilation",
   "wasi-nn",
-  "memfd",
   "pooling-allocator",
 ]
 jitdump = ["wasmtime/jitdump"]