Remove some dead code.

We currently have both `FromClean<clean::Constant> for Constant` and
`FromClean<clean::ConstantKind> for Constant` which are basically
identical, but the former is unused.

Author: nnethercote

src/librustdoc/clean/types.rs

@@ -2450,20 +2450,6 @@ pub(crate) enum ConstantKind {
     Infer,
 }
 
-impl Constant {
-    pub(crate) fn expr(&self, tcx: TyCtxt<'_>) -> String {
-        self.kind.expr(tcx)
-    }
-
-    pub(crate) fn value(&self, tcx: TyCtxt<'_>) -> Option<String> {
-        self.kind.value(tcx)
-    }
-
-    pub(crate) fn is_literal(&self, tcx: TyCtxt<'_>) -> bool {
-        self.kind.is_literal(tcx)
-    }
-}
-
 impl ConstantKind {
     pub(crate) fn expr(&self, tcx: TyCtxt<'_>) -> String {
         match *self {

src/librustdoc/json/conversions.rs

@@ -199,17 +199,6 @@ impl FromClean<clean::GenericArg> for GenericArg {
     }
 }
 
-impl FromClean<clean::Constant> for Constant {
-    // FIXME(generic_const_items): Add support for generic const items.
-    fn from_clean(constant: &clean::Constant, renderer: &JsonRenderer<'_>) -> Self {
-        let tcx = renderer.tcx;
-        let expr = constant.expr(tcx);
-        let value = constant.value(tcx);
-        let is_literal = constant.is_literal(tcx);
-        Constant { expr, value, is_literal }
-    }
-}
-
 impl FromClean<clean::ConstantKind> for Constant {
     // FIXME(generic_const_items): Add support for generic const items.
     fn from_clean(constant: &clean::ConstantKind, renderer: &JsonRenderer<'_>) -> Self {