Revert hooking into VimEnter

This mainly reverts the hooking into VimEnter (https://github.com/bogado/file-line/commit/f74d3f55), which appears to be meant as a workaround for #11.

The proper fix for #11 appears to be using `bufhidden=wipe`, instead of wiping the buffer during the BufNewFile event, which Vim does not appear to handle properly / as expected.