AK+Libraries: Remove FixedMemoryStream::[readonly_]bytes()

These methods are slightly more convenient than storing the Bytes
separately. However, it it feels unsanitary to reach in and access this
data directly. Both of the users of these already have the
[Readonly]Bytes available in their constructors, and can easily avoid
using these methods, so let's remove them entirely.

AK/MemoryStream.cpp

@@ -109,16 +109,6 @@ ErrorOr<void> FixedMemoryStream::write_until_depleted(ReadonlyBytes bytes)
     return {};
 }
 
-Bytes FixedMemoryStream::bytes()
-{
-    VERIFY(m_writing_enabled);
-    return m_bytes;
-}
-ReadonlyBytes FixedMemoryStream::readonly_bytes() const
-{
-    return m_bytes;
-}
-
 size_t FixedMemoryStream::offset() const
 {
     return m_offset;