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

@@ -140,9 +140,5 @@ 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"