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.

src/nvim/buffer_defs.h

@@ -582,7 +582,6 @@ struct file_buffer {
   char_u      *b_p_indk;        /* 'indentkeys' */
   char_u      *b_p_fex;         /* 'formatexpr' */
   long_u b_p_fex_flags;         /* flags for 'formatexpr' */
-  char_u      *b_p_key;         /* 'key' */
   char_u      *b_p_kp;          /* 'keywordprg' */
   int b_p_lisp;                 /* 'lisp' */
   char_u      *b_p_mps;         /* 'matchpairs' */
@@ -674,8 +673,6 @@ struct file_buffer {
   dictitem_T b_bufvar;          /* variable for "b:" Dictionary */
   dict_T      *b_vars;          /* internal variables, local to buffer */
 
-  char_u      *b_p_cm;          /* 'cryptmethod' */
-
   /* When a buffer is created, it starts without a swap file.  b_may_swap is
    * then set to indicate that a swap file may be opened later.  It is reset
    * if a swap file could not be opened.