[chore][exporter/clickhouse] Remove an unused function (#42811)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue. Ex. Adding a feature - Explain what this achieves.--> #### Description Removed the function `newPlaceholder` in the internal package as it seems no longer used. <!--Describe what testing was performed and which tests were added.--> #### Testing Unit & Integration tests

Author: gnak-yar

exporter/clickhouseexporter/internal/metrics/metrics_model.go

@@ -8,7 +8,6 @@ import (
 "encoding/hex"
 "errors"
 "fmt"
-"strings"
 "sync"
 
 "github.com/ClickHouse/clickhouse-go/v2"
@@ -204,13 +203,3 @@ func convertValueAtQuantile(valueAtQuantile pmetric.SummaryDataPointValueAtQuant
 }
 return quantiles, values
 }
-
-func newPlaceholder(count int) *string {
-var b strings.Builder
-for i := 0; i < count; i++ {
-b.WriteString(",?")
-}
-b.WriteString("),")
-placeholder := strings.Replace(b.String(), ",", "(", 1)
-return &placeholder
-}

exporter/clickhouseexporter/internal/metrics/metrics_model_test.go

@@ -222,11 +222,6 @@ func Test_getValue(t *testing.T) {
 })
 }
 
-func Test_newPlaceholder(t *testing.T) {
-expectStr := "(?,?,?,?,?),"
-require.Equal(t, newPlaceholder(5), &expectStr)
-}
-
 func Test_GetServiceName(t *testing.T) {
 t.Run("should return empty string on unset service.name", func(t *testing.T) {
 require.Empty(t, GetServiceName(pcommon.NewMap()))