vim: unset $TERM in GUIs for fzf on Windows

Workaround for junegunn/fzf#1677 (comment)

Author: janlazo

shared.vim

@@ -509,6 +509,7 @@ if has('autocmd')
     if s:is_gui
       set mouse=a
 
+      let $TERM = ''
       if $ConEmuANSI ==# 'ON'
         let $ConEmuANSI = 'OFF'
       endif