Remove the leftover usage of `posix-signals-on-macos` (#7360)

This follows up #6807 and removes the last remaining reference to
the removed `posix-signals-on-macos` feature flag.

Note that `lib.rs` now imports `mod unix` on MacOS. This change
is similar to the change in `traphandlers.rs` in #6807. It is
needed for hosts that use signals instead of Mach ports on MacOs.

crates/wasmtime/src/lib.rs

@@ -476,9 +476,7 @@ pub use anyhow::{Error, Result};
 pub mod component;
 
 cfg_if::cfg_if! {
-    if #[cfg(all(target_os = "macos", not(feature = "posix-signals-on-macos")))] {
-        // no extensions for macOS at this time
-    } else if #[cfg(unix)] {
+    if #[cfg(unix)] {
         pub mod unix;
     } else if #[cfg(windows)] {
         pub mod windows;