Remove the safe_suggestion feature

This removes the safe_suggestion feature from feature_gate.rs.
It was added in commit 164f010
and then removed again in commit c11fe55 .

As the removal was in the same PR rust-lang#38099 as the addition, we don't move it to
the "removed" section.

Removes an element from the whitelist of non gate tested unstable lang features (issue rust-lang#39059).

src/libsyntax/feature_gate.rs

@@ -313,9 +313,6 @@ declare_features! (
     // Allows #[target_feature(...)]
     (active, target_feature, "1.15.0", None),
 
-    // Allow safe suggestions for potential type conversions.
-    (active, safe_suggestion, "1.0.0", Some(37384)),
-
     // `extern "ptx-*" fn()`
     (active, abi_ptx, "1.15.0", None),
 

src/tools/tidy/src/features.rs

@@ -165,7 +165,7 @@ pub fn check(path: &Path, bad: &mut bool) {
 
     // FIXME get this whitelist empty.
     let whitelist = vec![
-        "abi_ptx", "simd", "safe_suggestion", "macro_reexport",
+        "abi_ptx", "simd", "macro_reexport",
         "more_struct_aliases", "static_recursion", "reflect",
         "quote", "cfg_target_has_atomic", "custom_attribute",
         "default_type_parameter_fallback", "pushpop_unsafe",