LibAudio: Remove unnecessary m_bitstream field from MP3Loader

We can just create a `BigEndianInputBitStream` where it is needed
instead of storing one in the class. After making `read_header()`
static, the only other user of the field was `read_size_information()`,
so let's do that there and then remove the field.

Userland/Libraries/LibAudio/MP3Loader.h

@@ -72,7 +72,6 @@ private:
     int m_total_samples { 0 };
     size_t m_loaded_samples { 0 };
 
-    OwnPtr<BigEndianInputBitStream> m_bitstream;
     AllocatingMemoryStream m_bit_reservoir;
 };