remove the unused 'const-fn' feature

Cargo.toml

@@ -13,5 +13,4 @@ bare-metal = "0.2.0"
 bit_field = "0.9.0"
 
 [features]
-const-fn = ["bare-metal/const-fn"]
 inline-asm = []

ci/script.sh

@@ -4,7 +4,7 @@ main() {
     cargo check --target $TARGET
 
     if [ $TRAVIS_RUST_VERSION = nightly ]; then
-        cargo check --target $TARGET --features 'const-fn inline-asm'
+        cargo check --target $TARGET --features inline-asm
     fi
 
     if [ $TARGET = x86_64-unknown-linux-gnu ]; then