rustc: remove ArgSource

`ArgSource` is no longer used anywhere, so it can be removed.

Author: davidtwco

src/librustc/hir/lowering.rs

@@ -2285,7 +2285,6 @@ impl<'a> LoweringContext<'a> {
         hir::Arg {
             hir_id: self.lower_node_id(arg.id),
             pat: self.lower_pat(&arg.pat),
-            source: hir::ArgSource::Normal,
         }
     }
 
@@ -3091,7 +3090,6 @@ impl<'a> LoweringContext<'a> {
                 let new_argument = hir::Arg {
                     hir_id: argument.hir_id,
                     pat: new_argument_pat,
-                    source: hir::ArgSource::AsyncFn
                 };
 
                 if is_simple_argument {

src/librustc/hir/mod.rs

@@ -1929,16 +1929,6 @@ pub struct InlineAsm {
 pub struct Arg {
     pub pat: P<Pat>,
     pub hir_id: HirId,
-    pub source: ArgSource,
-}
-
-/// Represents the source of an argument in a function header.
-#[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
-pub enum ArgSource {
-    /// Argument as specified by the user.
-    Normal,
-    /// Generated argument from `async fn` lowering.
-    AsyncFn,
 }
 
 /// Represents the header (not the body) of a function declaration.