Deduplicate LOG_EVENT_OFFSET

This discussion found that LOG_EVENT_OFFSET is a duplicate of EVENT_TYPE_OFFSET and BIN_LOG_HEADER_SIZE depending on the context.

• BIN_LOG_HEADER_SIZE
  • The size of the "0xfe""bin" magic number
  • Binlog Events § Log File Structure
• EVENT_TYPE_OFFSET
  • The index of the event type in an event
  • Binlog Event Header § Event Header Structure