CRYPTO: Do not call NSS_Shutdown after every operation
    
    Calling setup and teardown on every encryption cases issues like the one
    described in https://bugzilla.redhat.com/show_bug.cgi?id=1456151
    eventually.
    
    Similarly to other crypto functions, don't tear down NSS by calling
    NSS_Shutdown. Let the OS reclaim the resources.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3424
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>