remove passing privateKey to RemoteSecondary constructor

[murali-shris]

Describe the bug

Private key shouldn't be read from preferences anywhere in the atClient code

Steps to reproduce

n/a

Expected behavior

#1141
atsign-foundation/at_libraries#434
Once the above tasks are done, we can remove code which read private key from preference

• AtClientImpl --> _init(..) remove privateKey from being passed to RemoteSecondary constructor
  _remoteSecondary ??= RemoteSecondary(_atSign, _preference!,
  atChops: atChops,
  privateKey: _preference!.privateKey,
  enrollmentId: _enrollmentId);
• RemoteSecondary constructor
  Remove the line
  privateKey ??= preference.privateKey;
  and remove passing of privateKey to AtLookupImpl constructor
  atLookUp = AtLookupImpl(atSign, preference.rootDomain, preference.rootPort,
  privateKey: privateKey,
  cramSecret: preference.cramSecret,
  secondaryAddressFinder:
  AtClientManager.getInstance().secondaryAddressFinder,
  secureSocketConfig: secureSocketConfig,
  clientConfig: _getClientConfig());

Screenshots

No response

Smartphones

No response

Were you using an atApplication when the bug was found?

No response

Additional context

No response