rustc_parse: Remove unused dependency smallvec

Unused since commit 530a629
("Remove pretty-print/reparse hack, and add derive-specific hack").

Author: joshtriplett

Cargo.lock

@@ -4182,7 +4182,6 @@ dependencies = [
  "rustc_lexer",
  "rustc_session",
  "rustc_span",
- "smallvec",
  "tracing",
  "unicode-normalization",
 ]

compiler/rustc_parse/Cargo.toml

@@ -19,4 +19,3 @@ rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_ast = { path = "../rustc_ast" }
 unicode-normalization = "0.1.11"
-smallvec = { version = "1.6.1", features = ["union", "may_dangle"] }