chore: Remove no_btreemap_retain feature from build.rs

build.rs

@@ -33,12 +33,6 @@ fn main() {
     if minor < 45 {
         println!("cargo:rustc-cfg=no_btreemap_remove_entry");
     }
-
-    // BTreeMap::retain
-    // https://blog.rust-lang.org/2021/06/17/Rust-1.53.0.html#stabilized-apis
-    if minor < 53 {
-        println!("cargo:rustc-cfg=no_btreemap_retain");
-    }
 }
 
 fn rustc_minor_version() -> Option<u32> {

src/map.rs

@@ -276,7 +276,6 @@ impl Map<String, Value> {
     ///
     /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)`
     /// returns `false`.
-    #[cfg(not(no_btreemap_retain))]
     #[inline]
     pub fn retain<F>(&mut self, f: F)
     where

tests/map.rs

@@ -35,7 +35,6 @@ fn test_append() {
     assert!(val.is_empty());
 }
 
-#[cfg(not(no_btreemap_retain))]
 #[test]
 fn test_retain() {
     let mut v: Value = from_str(r#"{"b":null,"a":null,"c":null}"#).unwrap();