certificate: Allow deleting LDAP client certificate

Users should be able to delete LDAP client certificate, and phosphor-
certificate-manager already supports this. This patch adds the Redfish
API endpoint for it.

Tested:
DELETE /redfish/v1/AccountService/LDAP/Certificates/1 removes the LDAP
client certificate installed on system.

Change-Id: If1c082c78c404b5877e74ba50de3a970cd823a90
Signed-off-by: Jiaqing Zhao <jiaqing.zhao@intel.com>