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/memfile_defs.h

@@ -75,8 +75,6 @@ struct nr_trans {
   blocknr_T nt_new_bnum;                /* new, positive, number */
 };
 
-#define MF_SEED_LEN     8
-
 struct memfile {
   char_u      *mf_fname;                /* name of the file */
   char_u      *mf_ffname;               /* idem, full path */
@@ -94,14 +92,6 @@ struct memfile {
   blocknr_T mf_infile_count;            /* number of pages in the file */
   unsigned mf_page_size;                /* number of bytes in a page */
   int mf_dirty;                         /* TRUE if there are dirty blocks */
-  buf_T       *mf_buffer;               /* bufer this memfile is for */
-  char_u mf_seed[MF_SEED_LEN];          /* seed for encryption */
-
-  /* Values for key, method and seed used for reading data blocks when
-   * updating for a newly set key or method. Only when mf_old_key != NULL. */
-  char_u      *mf_old_key;
-  int mf_old_cm;
-  char_u mf_old_seed[MF_SEED_LEN];
 };
 
 #endif // NVIM_MEMFILE_DEFS_H