parser: fix parsing of modifiers for let definitions

examples/count.pork

@@ -1,4 +1,4 @@
-let count = 5
+export let count = 5
 
 export func main() {
   var x = 1

parser/src/main/kotlin/gay/pizza/pork/parser/Parser.kt

@@ -266,7 +266,6 @@ class Parser(source: PeekableSource<Token>, val attribution: NodeAttribution) {
   }
 
   private fun readLetDefinition(modifiers: DefinitionModifiers): LetDefinition = within {
-    val modifiers = readDefinitionModifiers()
     expect(TokenType.Let)
     val name = readSymbolRaw()
     expect(TokenType.Equals)