clippy: fix manual_let_else lint

Cargo.toml

@@ -152,7 +152,6 @@ used_underscore_binding = "allow"            # 2
 too_many_lines = "allow"                     # 2
 struct_excessive_bools = "allow"             # 2
 match_same_arms = "allow"                    # 2
-manual_let_else = "allow"                    # 2
 manual_assert = "allow"                      # 2
 ignore_without_reason = "allow"              # 2
 format_push_string = "allow"                 # 2

src/sed/fast_io.rs

@@ -630,10 +630,9 @@ impl OutputBuffer {
             self.flush_mmap(WriteRange::Complete)?;
         }
 
-        let file = match File::open(path) {
-            Ok(f) => f,
+        let Ok(file) = File::open(path) else {
             // Per POSIX, if the file can't be read treat it as empty.
-            Err(_) => return Ok(()),
+            return Ok(());
         };
 
         let mut reader = BufReader::new(file);