feat: allow upsert existing basic connection (#2181)

## Describe your changes

Following our conversation with Thomas, we agree that we can allow
upsert existing basic connection.

It is already allowed for Oauth2 and ApiKey connection but not for basic
credentials.

This PR remove the check which disallow the import of an existing
connection with basic credentials.

## Issue ticket number and link

## Checklist before requesting a review (skip if just adding/editing
APIs & templates)
- [ ] I added tests, otherwise the reason is: 
- [ ] I added observability, otherwise the reason is:
- [ ] I added analytics, otherwise the reason is:

packages/shared/lib/services/connection.service.ts

@@ -238,12 +238,6 @@ class ConnectionService {
         credentials: BasicApiCredentials | ApiKeyCredentials,
         connectionCreatedHook: (res: ConnectionUpsertResponse) => Promise<void>
     ) {
-        const connection = await this.checkIfConnectionExists(connection_id, provider_config_key, environmentId);
-
-        if (connection) {
-            throw new NangoError('connection_already_exists');
-        }
-
         const [importedConnection] = await this.upsertApiConnection(connection_id, provider_config_key, provider, credentials, {}, environmentId, accountId);
 
         if (importedConnection) {