chore: remove unnecessary mutex

Author: tstirrat15

balancer.go

@@ -16,7 +16,6 @@ import (
 	"errors"
 	"fmt"
 	"hash/maphash"
-	"sync"
 
 	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/balancer/base"
@@ -128,7 +127,6 @@ func (s subConnMember) Key() string { return s.key }
 var _ hashring.Member = (*subConnMember)(nil)
 
 type builder struct {
-	sync.Mutex
 	hashfn hashring.HashFunc
 	config BalancerConfig
 }