Remove the need to wait for target block header in warp sync implementation (#5431)

I'm not sure if this is exactly what
https://github.com/paritytech/polkadot-sdk/issues/3537 meant, but I
think it should be fine to wait for relay chain before initializing
parachain node fully, which removed the need for background task and
extra hacks throughout the stack just to know where warp sync should
start.

Previously there were both `WarpSyncParams` and `WarpSyncConfig`, but
there was no longer any point in having two data structures, so I
simplified it to just `WarpSyncConfig`.

Fixes https://github.com/paritytech/polkadot-sdk/issues/3537