Vim plugin doesn't work at all on Cygwin; removing special-casing in fzf.vim may fix it

- [X] I have read through the manual page (`man fzf`)
- [X] I have the latest version of fzf
- [X] I have searched through the existing issues

## Info

- OS
- [ ] Linux
- [ ] Mac OS X
- [X] Windows
- [ ] Etc.
- Shell
- [ ] bash
- [ ] zsh
- [ ] fish

## Problem / Steps to reproduce

I am running Vim with the fzf and fzf.vim plugins under Cygwin. Whenever I try to use one of the fzf.vim commands, such as `:Files` or `:Rg`, the following happens:

1. The terminal goes blank
2. A separate console window opens and closes again immediately. (No output seems to appear in it.)
3. The terminal un-blanks again, and I'm back where I started (e.g. editing a file).

All of this happens in about a second.

I've found that by commenting out the following `elseif` block [at line 590 of `plugin/fzf.vim`](https://github.com/junegunn/fzf/blob/c0a83b27eb0e2a9a1f631d04830a7dc6d762a951/plugin/fzf.vim#L590) in its entirety, I can restore what seems to be the correct behaviour, namely that the fzf file selection pane is shown:
```vim
elseif has('win32unix') && $TERM !=# 'cygwin'
let shellscript = s:fzf_tempname()
call writefile([command], shellscript)
let command = 'cmd.exe /C '.fzf#shellescape('set "TERM=" & start /WAIT sh -c '.shellscript)
let a:temps.shellscript = shellscript
```
It seems to be treating Cygwin differently from other Windows-UNIX environments, and I'm wondering whether that's still the right thing to do. Cygwin now uses mintty as its default terminal, and that has `TERM=xterm` by default. In fact, `TERM=cygwin` doesn't even seem to be an option in mintty.