LibGfx/ISOBMFF: Remove prototypes for nonexistent methods

Userland/Libraries/LibGfx/ImageFormats/ISOBMFF/Reader.h

@@ -19,9 +19,6 @@ public:
 
     ErrorOr<BoxList> read_entire_file();
 
-    ErrorOr<BrandIdentifier> get_major_brand();
-    ErrorOr<Vector<BrandIdentifier>> get_minor_brands();
-
 private:
     Reader(MaybeOwned<SeekableStream> stream, size_t size)
         : m_stream(move(stream))
@@ -29,8 +26,6 @@ private:
     {
     }
 
-    ErrorOr<void> parse_initial_data();
-
     MaybeOwned<SeekableStream> m_stream;
     BoxStream m_box_stream;
 };