uv-resolver: simplify `fork_markers` construction

This doesn't change any behavior. My guess is that this code was
a casualty of refactoring. But basically, it was doing redundant
case analysis and iterating over all resolutions (even though it's
in the branch that can only occur when there is only one
resolution).

crates/uv-resolver/src/resolution/output.rs

@@ -174,33 +174,12 @@ impl ResolverOutput {
         // Extract the `Requires-Python` range, if provided.
         let requires_python = python.target().clone();
 
-        let fork_markers = if let [resolution] = resolutions {
+        let fork_markers: Vec<MarkerTree> = if let [resolution] = resolutions {
-            resolution
+            resolution.env.try_markers().cloned().into_iter().collect()
-                .env
-                .try_markers()
-                .map(|_| {
-                    resolutions
-                        .iter()
-                        .map(|resolution| {
-                            resolution
-                                .env
-                                .try_markers()
-                                .expect("A non-forking resolution exists in forking mode")
-                                .clone()
-                        })
-                        .collect()
-                })
-                .unwrap_or_else(Vec::new)
         } else {
             resolutions
                 .iter()
-                .map(|resolution| {
+                .map(|resolution| resolution.env.try_markers().cloned().unwrap_or_default())
-                    resolution
-                        .env
-                        .try_markers()
-                        .cloned()
-                        .unwrap_or(MarkerTree::TRUE)
-                })
                 .collect()
         };