cleanup: Remove the const-extern-fn feature

Since moving from a feature to a `cfg`, this feature is unneeded. Remove
it in 1.0.

Not intended for backport.

Cargo.toml

@@ -135,14 +135,10 @@ cargo-args = ["-Zbuild-std=core"]
 rustc-std-workspace-core = { version = "1.0.0", optional = true }
 
 [features]
-default = ["const-extern-fn", "std"]
+default = ["std"]
 std = []
 rustc-dep-of-std = ["rustc-std-workspace-core"]
 extra_traits = []
 
-# `const-extern-function` is deprecated and no longer does anything
-# FIXME(1.0): remove this completely
-const-extern-fn = []
-
 [workspace]
 members = ["libc-test"]

ci/verify-build.sh

@@ -62,7 +62,6 @@ test_target() {
 
     # Test with expected combinations of features
     $cmd
-    $cmd --features const-extern-fn
     $cmd --features extra_traits
 
     # Test again without default features, i.e. without "std"

src/macros.rs

@@ -169,7 +169,7 @@ macro_rules! e {
 
 // This is a pretty horrible hack to allow us to conditionally mark
 // some functions as 'const', without requiring users of this macro
-// to care about the "const-extern-fn" feature.
+// to care about the "const-extern-fn" feature (or `const_extern_fn` config).
 //
 // When 'const-extern-fn' is enabled, we emit the captured 'const' keyword
 // in the expanded function.