feat(vim): remove single plug thread limitation
feat(vim): remove single plug thread limitation

This was added in 4a2414c in the hope of fixing random
segfaults/crashes, as described in [1]. But the crashes continue, so
this is likely not the cause.

Remaining possible causes?/references: [2], [3], [4]. But maybe not
because my current vim version at time or writing is 7.4.1-663, which is
supposed to have the patch as referenced in [3] and [4]...?

[1]: https://github.com/junegunn/vim-plug/wiki/faq#vim-caught-deadly-signal-segv
[2]: https://github.com/Shougo/neocomplete.vim/issues/310
[3]: https://groups.google.com/forum/#!searchin/vim_dev/GC/vim_dev/DBYOdHQWvqY/1WH04_dwETIJ
[4]: https://gist.github.com/mattn/0c58a7398c63ab4c3066

vyp/ui,vyp/etsi