LibPDF: Get rid of PlainText/Encoded StreamObject

This was a small optimization to allow a stream object to simply hold
a reference to the bytes in a PDF document rather than duplicating
them. However, as we move into features such as encryption, this
optimization does more harm than good. This can be revisited in the
future if necessary.

Userland/Libraries/LibPDF/Forward.h

@@ -14,9 +14,6 @@ namespace PDF {
 class Document;
 class Object;
 
-// Note: This macro doesn't care about PlainTextStreamObject and EncodedStreamObject because
-//       we never need to work directly with either of them.
-
 #define ENUMERATE_OBJECT_TYPES(V) \
     V(StringObject, string)       \
     V(NameObject, name)           \