[RDY] Remove cryptography

From the discussion in #694 there seemed to be overwhelming support to get rid of the built-in cryptography in neovim. Here's my plan:

- [x] Remove cryptography-related files.
- [x] Remove all code between `FEAT_CRYPT` ifdefs in the original vim code base on a file by file basis.
- [x] Fix any broken tests
- [x] Update documentation.
- [x] Remove leftovers in memline.c

Comments / suggestions?