Remove nss libk5crypto implementation

Remove nss libk5crypto implementation

Delete the nss libk5crypto implementation because: (1) the original
contributor no longer requires it; (2) as far as we know it is rarely
or never used; (3) NSS has significant impedance mismatch with MIT
krb5's requirements, such as the inability to preserve crypto state
across fork(); (4) NSS cannot do PBKDF2 with SHA-256 or SHA-384, as
will be required for the aes-sha2 enctypes.

ticket: 8345 (new)