Remove unused SyntaxContext serialization impls

The implementations were wrong and unused.

src/librustc_metadata/decoder.rs

@@ -356,14 +356,6 @@ impl SpecializedDecoder<Ident> for DecodeContext<'_, '_> {
     }
 }
 
-impl SpecializedDecoder<SyntaxContext> for DecodeContext<'_, '_> {
-    fn specialized_decode(&mut self) -> Result<SyntaxContext, Self::Error> {
-        // FIXME(jseyfried): intercrate hygiene
-
-        Ok(SyntaxContext::empty())
-    }
-}
-
 impl<'a, 'tcx> SpecializedDecoder<Fingerprint> for DecodeContext<'a, 'tcx> {
     fn specialized_decode(&mut self) -> Result<Fingerprint, Self::Error> {
         Fingerprint::decode_opaque(&mut self.opaque)

src/librustc_metadata/encoder.rs

@@ -33,7 +33,6 @@ use syntax::attr;
 use syntax::source_map::Spanned;
 use syntax::symbol::{kw, sym, Ident};
 use syntax_pos::{self, FileName, SourceFile, Span};
-use syntax_pos::hygiene::SyntaxContext;
 use log::{debug, trace};
 
 use rustc::hir::{self, PatKind};
@@ -181,13 +180,6 @@ impl SpecializedEncoder<Ident> for EncodeContext<'tcx> {
     }
 }
 
-impl SpecializedEncoder<SyntaxContext> for EncodeContext<'tcx> {
-    fn specialized_encode(&mut self, _ctxt: &SyntaxContext) -> Result<(), Self::Error> {
-        // FIXME(jseyfried): intercrate hygiene
-        Ok(())
-    }
-}
-
 impl<'tcx> SpecializedEncoder<LocalDefId> for EncodeContext<'tcx> {
     #[inline]
     fn specialized_encode(&mut self, def_id: &LocalDefId) -> Result<(), Self::Error> {

src/libsyntax_pos/hygiene.rs

@@ -750,7 +750,3 @@ impl Decodable for ExpnId {
         Ok(ExpnId::root()) // FIXME(jseyfried) intercrate hygiene
     }
 }
-
-impl UseSpecializedEncodable for SyntaxContext {}
-
-impl UseSpecializedDecodable for SyntaxContext {}