Remove duplicate should_fetch_blobs check in sync lookup

[dapplion]

Issue Addressed

The function blob_lookup_request now includes two checks if a block is in the da window:

• First: check if clock slot is in da_window
• Second: check if block's slot is in da_window

https://github.com/sigp/lighthouse/blob/5a966874da03cd987fef1629f732898a2859ce87/beacon_node/network/src/sync/network_context.rs#L641-L643

The first check has been there since pre-deneb, and is incorrect da_window-inclusiveness should be computed on the block slot not the clock.

Proposed Changes

Remove the first check