Remove faulty Identifier.toAstIdentifier() method

Contrary to what the docstring says ("Identifier ready to be used in KS
expressions."), nothing could be further from the truth since it relies
on `humanReadable`, which is (as the name suggests) merely a
*human-readable* identifier, i.e. completely unusable for the purpose
stated here.

For this reason, using the `toAstIdentifier()` method in a hope that it
would work as declared often led to bugs, because for example it didn't
work for unnamed fields (`NumberedIdentifier`), as they normally cannot
appear in user-supplied expressions. Fortunately, all former usages of
`toAstIdentifier()` were already changed in commit e535b20 to use
`Ast.expr.InternalName`, which was specifically designed for the purpose
of injecting any identifiers we need (including unnamed fields) when
crafting an AST internally in the compiler. So we can finally remove
`toAstIdentifier()` for good because it is no longer used.

shared/src/main/scala/io/kaitai/struct/format/Identifier.scala

@@ -12,11 +12,6 @@ abstract class Identifier {
     *         error messaging purposes.
     */
   def humanReadable: String
-
-  /**
-    * @return Identifier ready to be used in KS expressions.
-    */
-  def toAstIdentifier: Ast.identifier = Ast.identifier(humanReadable)
 }
 
 /**