Remove ''${ escape sequence for double-quoted strings (#158)

The purpose of this change is to match this change to the grammar:

https://github.com/dhall-lang/dhall-lang/pull/46

src/Dhall/Parser.hs

@@ -277,7 +277,6 @@ doubleQuotedChunk :: Parser a -> Parser (Expr Src a)
 doubleQuotedChunk embedded =
     choice
         [ interpolation
-        , escapeInterpolation
         , unescapedCharacter
         , escapedCharacter
         ]
@@ -288,10 +287,6 @@ doubleQuotedChunk embedded =
         _ <- Text.Parser.Char.char '}'
         return e
 
-    escapeInterpolation = do
-        _ <- Text.Parser.Char.text "''${"
-        return (TextLit "${")
-
     unescapedCharacter = do
         c <- Text.Parser.Char.satisfy predicate
         return (TextLit (Data.Text.Lazy.Builder.singleton c))

tests/parser/escapedDoubleQuotedString.dhall

@@ -1 +1 @@
-"''${\\\"\$\\\/\b\f\n\r\t \u2200(a : Type) \u2192 a"
+"\\\"\$\\\/\b\f\n\r\t \u2200(a : Type) \u2192 a"