rustc_session: Remove unused dependency bitflags

Unused since commit 64af7ea
("Move SanitizerSet to rustc_target").

Author: joshtriplett

Cargo.lock

@@ -4341,7 +4341,6 @@ dependencies = [
 name = "rustc_session"
 version = "0.0.0"
 dependencies = [
- "bitflags",
  "getopts",
  "num_cpus",
  "rustc_ast",

compiler/rustc_session/Cargo.toml

@@ -5,7 +5,6 @@ version = "0.0.0"
 edition = "2018"
 
 [dependencies]
-bitflags = "1.2.1"
 getopts = "0.2"
 rustc_macros = { path = "../rustc_macros" }
 tracing = "0.1"