Remove 'unstable' cfg

Author: dtolnay

Cargo.toml

@@ -21,13 +21,9 @@ doctest = false
 unicode-xid = "0.1"
 
 [features]
-
 # When enabled: act as a shim around the nightly compiler's proc_macro crate.
 # This requires a nightly compiler.
 #
 # When disabled: emulate the same API as the nightly compiler's proc_macro crate
 # but in a way that works on all stable compilers >=1.15.0.
 nightly = []
-
-# Deprecated; use "nightly" instead.
-unstable = ["nightly"]