beacon/light: remove unused CommitteeChain.signerThreshold (#30484)

This field is a duplicate of UpdateScore.SignerCount and never referenced.

beacon/light/committee_chain.go

@@ -77,7 +77,6 @@ type CommitteeChain struct {
 	sigVerifier committeeSigVerifier // BLS sig verifier (dummy verifier in tests)
 
 	config             *types.ChainConfig
-	signerThreshold    int
 	minimumUpdateScore types.UpdateScore
 	enforceTime        bool // enforceTime specifies whether the age of a signed header should be checked
 }
@@ -102,7 +101,6 @@ func newCommitteeChain(db ethdb.KeyValueStore, config *types.ChainConfig, signer
 		clock:          clock,
 		unixNano:       unixNano,
 		config:         config,
-		signerThreshold: signerThreshold,
 		enforceTime:    enforceTime,
 		minimumUpdateScore: types.UpdateScore{
 			SignerCount:    uint32(signerThreshold),