Add clear_keycredentiallinks to ldap_shell

[rtpt-romankarwacik]

Very similar to do_clear_rbcd. In a Shadow-Credential-Attack involving a computer account, the keycredentiallinks needs to be empty currently in ntlmrelayx, as the new keycredential is just added to the existing one, and only one keycredentiallinks can exist for a computers own object, so this can be used to clear it beforehand. This should be done instead of just clearing it in the shadow credential attack in ntlmrelayx, as corrupting the keycredentials could disrupt the computers functionality. Instead, The keycredentials should be backuped before, and restored after the attack has been conducted.