refactor: Remove unused createCustomersCollection export and related usage

- Eliminate `createCustomersCollection` from collections and main index files
- Update `config.collections` logic to remove customer collection dependency

src/collections/index.ts

@@ -1,4 +1,3 @@
-export { createCustomersCollection } from './customers'
 export { createInvoicesCollection } from './invoices'
 export { createPaymentsCollection } from './payments'
 export { createRefundsCollection } from './refunds'

src/index.ts

@@ -2,7 +2,6 @@ import type { Config } from 'payload'
 
 import type { BillingPluginConfig, CustomerInfoExtractor } from './types'
 
-import { createCustomersCollection } from './collections/customers'
 import { createInvoicesCollection } from './collections/invoices'
 import { createPaymentsCollection } from './collections/payments'
 import { createRefundsCollection } from './collections/refunds'
@@ -42,7 +41,6 @@ export const billingPlugin = (pluginConfig: BillingPluginConfig = {}) => (config
 
   config.collections.push(
     createPaymentsCollection(pluginConfig.collections?.payments || 'payments'),
-    createCustomersCollection(customerSlug),
     createInvoicesCollection(
       pluginConfig.collections?.invoices || 'invoices',
       pluginConfig.collections?.customerRelation !== false ? customerSlug : undefined,