Fix: remove phase0 redundant check

[Alleysira]

Description

Hi there,

I'm studying consensus-spec recently and found that the beacon_aggregate_and_proof gossip validation in specs/phase0/p2p-interface.md contains two identical [REJECT] checks that test the exact same condition with the same inputs.

consensus-specs/specs/phase0/p2p-interface.md

Lines 513 to 514 in 1baa05e

	- _[REJECT]_ The aggregate attestation has participants -- that is,
	`len(get_attesting_indices(state, aggregate)) >= 1`.

and

consensus-specs/specs/phase0/p2p-interface.md

Lines 522 to 523 in 1baa05e

	- _[REJECT]_ The attestation has participants -- that is,
	`len(get_attesting_indices(state, aggregate)) >= 1`.

This PR removes the second check and keep the first which uses the more specific aggregate attestation wording consistent with the surrounding context.

[jihoonsong]

Hi @Alleysira, thanks for your contribution. I just wanted to let you know that this will be removed and replaced with code in #4902. Please understand that I'm closing this PR in lieu of #4902. Feel free to continue your study and further contribution :)