subscriber: don't gate with_ansi() on the "ansi" feature (#3020)

The commit 1cb523b removed this cfg gate on master. However, when
the change was backported in 1cb523b the docs were updated but the
cfg change was omitted.

This made the docs misleading, since they say "This method itself is
still available without the feature flag."

tracing-subscriber/src/fmt/mod.rs

@@ -627,8 +627,6 @@ where
     /// ANSI escape codes can ensure that they are not used, regardless of
     /// whether or not other crates in the dependency graph enable the "ansi"
     /// feature flag.
-    #[cfg(feature = "ansi")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "ansi")))]
     pub fn with_ansi(self, ansi: bool) -> SubscriberBuilder<N, format::Format<L, T>, F, W> {
         SubscriberBuilder {
             inner: self.inner.with_ansi(ansi),