fix(payments): stop doubling the pack size in buyer mail

Catalog titles already name the pack ("50 фишек"), so appending the chip count
produced "50 фишек — 50 фишек, 79.00 ₽". The letter now carries the title
exactly as the storefront shows it, then the price.

The tax receipt is unaffected: it states the quantity in the wording the tax
service wants, which is a separate surface with separate requirements.

backend/internal/payments/mynalog.go

@@ -125,7 +125,6 @@ type PurchaseMail struct {
 	OrderID     uuid.UUID
 	Origin      string
 	Title       string
-	Chips       int
 	AmountMinor int64
 	Currency    string
 	CreatedAt   time.Time
@@ -137,7 +136,6 @@ type ReceiptMail struct {
 	AccountID     uuid.UUID
 	ReceiptUUID   string
 	Title         string
-	Chips         int
 	AmountMinor   int64
 	Currency      string
 	OperationTime time.Time