Remove cryptography

As discussed in #694, vim encryption uses old,
obsolete algorithms that are poorly implemented.
Since insecure cryptography is worse than no
cryptgraphy, the community voted in favor of
removing all crypto.

Various alternatives to the old crypto is
being discussed in #701.

Closes #694.
Merge pull request #699 'Remove cryptography'
Extract method from spell_load_lang()
Split if statement for better clarity
Extract method from slang_clear()