tracing-core: Do not add valuable/std feature as dependency unless …

…`valuable` is used (tokio-rs#3002)

## Motivation

`tracing-core` adds a dependency on `valuable` anytime the `std` (or `default`) feature is enabled even when `valuable` is not meant to be used. This was pointed out by [this comment](tokio-rs#1608 (comment)).

## Solution

Only add the feature dependency when something enables `valuable`.

This does not apply to `master` as `valuable` support there is always on.

tracing-core/Cargo.toml

@@ -27,7 +27,7 @@ edition = "2018"
 rust-version = "1.63.0"
 
 [features]
-default = ["std", "valuable/std"]
+default = ["std", "valuable?/std"]
 std = ["once_cell"]
 
 [badges]