Cleanup/Remove fast sync with header only

[asdacap]

• Remove blockdownloader capability to forward sync with header only or without downloading receipt when not processing.
  • This was somewhat useful before when sync pivot is not updated from CL, but now it make things hard to understand.
• Also try to reduce duplicated code between MergeBlockDownloader and BlockDownloader.
  • Cut down a lot of code.
  • Make it easier to fix other stuff like state sync being run multiple time or do things like make forward sync download in parallel.
• Fix/Disallow the unclear behaviour where if a node is synced with DownloadBodiesInFastSync or DownloadReceiptsInFastSync set to false, then after snap sync, restarted with the two option set to true, the portion of receipts that is after sync pivot is not downloaded.
• Fix receipts on fast sync transition edge case not working on non-merge network since it is only available on MergeBlockDownloader, not BlockDownloader.

Types of changes

What types of changes does your code introduce?

• Bugfix (a non-breaking change that fixes an issue)
• Refactoring

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

• Being tested....

[asdacap]

DownloadBlocks also have the beacon pivot check so this method is not needed.