subscriber: remove deprecated APIs (tokio-rs#1673)

## Motivation

`tracing-subscriber` currently contains some APIs that were deprecated
in the v0.2.x series:
- `fmt::LayerBuilder`, which is now a type alias for `fmt::Layer` (as
  the `Layer` type can expose all the same methods as the builder)
- `registry::SpanRef::parent_id`, which doesn't play nice with per-layer
  filtering,
- `fmt::Layer::inherit_fields`, which no longer does anything as it's
  now the default behavior
- `fmt::Layer::on_event`, which was renamed to `fmt_event`
- the `SpanRef::parents` and `layer::Context::scope` iterators, which
  were replaced by the APIs added in tokio-rs#1431 and tokio-rs#1434

Prior to releasing v0.3, the deprecated APIs should be removed.

## Solution

This branch deletes the deprecated APIs, with the exception of
`SpanRef::parents` and `Context::scope` (which were already removed in
240d11a).

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -72,34 +72,7 @@ pub struct Layer<
     _inner: PhantomData<S>,
 }
 
-/// A builder for [`Layer`](struct.Layer.html) that logs formatted representations of `tracing`
-/// events and spans.
-///
-/// **Note**: As of `tracing-subscriber` 0.2.4, the separate builder type is now
-/// deprecated, as the `Layer` type itself supports all the builder's
-/// configuration methods. This is now an alias for `Layer`.
-#[deprecated(
-    since = "0.2.4",
-    note = "a separate layer builder type is not necessary, `Layer`s now support configuration"
-)]
-pub type LayerBuilder<
-    S,
-    N = format::DefaultFields,
-    E = format::Format<format::Full>,
-    W = fn() -> io::Stdout,
-> = Layer<S, N, E, W>;
-
 impl<S> Layer<S> {
-    /// Returns a new [`LayerBuilder`](type.LayerBuilder.html) for configuring a `Layer`.
-    #[deprecated(
-        since = "0.2.4",
-        note = "a separate layer builder is not necessary, use `Layer::new`/`Layer::default` instead"
-    )]
-    #[allow(deprecated)]
-    pub fn builder() -> LayerBuilder<S> {
-        Layer::default()
-    }
-
     /// Returns a new [`Layer`](struct.Layer.html) with the default configuration.
     pub fn new() -> Self {
         Self::default()
@@ -475,26 +448,6 @@ impl<S, N, E, W> Layer<S, N, E, W> {
     }
 }
 
-#[allow(deprecated)]
-impl<S, N, E, W> LayerBuilder<S, N, E, W>
-where
-    S: Subscriber + for<'a> LookupSpan<'a>,
-    N: for<'writer> FormatFields<'writer> + 'static,
-    E: FormatEvent<S, N> + 'static,
-    W: for<'writer> MakeWriter<'writer> + 'static,
-{
-    /// Builds a [`Layer`] with the provided configuration.
-    ///
-    /// [`Layer`]: struct.Layer.html
-    #[deprecated(
-        since = "0.2.4",
-        note = "`LayerBuilder` is no longer a separate type; this method is not necessary"
-    )]
-    pub fn finish(self) -> Layer<S, N, E, W> {
-        self
-    }
-}
-
 impl<S> Default for Layer<S> {
     fn default() -> Self {
         Layer {

tracing-subscriber/src/registry/mod.rs

@@ -468,7 +468,6 @@ where
 
     /// Returns a `SpanRef` describing this span's parent, or `None` if this
     /// span is the root of its trace tree.
-
     pub fn parent(&self) -> Option<Self> {
         let id = self.data.parent()?;
         let data = self.registry.span_data(id)?;