Avoid printing an error response when detecting xclip.

While we're at, using the slightly more portable `command -v` technique
to detect the executable.  Also, there's no need to use `io.popen()` if
we aren't going to record the path.  Instead, let's use the simpler
`os.execute()` to detect the presence of xclip.
Merge pull request #1555 from jszakmeister/silent-xclip-find-executable

Avoid printing an error response when detecting xclip.
Remove remaining crypto references

Closes https://github.com/neovim/docs/pull/26

Also added stubs for 'cryptmethod' and 'key', and placeholders for
explanation regarding removal of crypto functionality.