Revert "plugin/magit.vim: simplify auto jump to magit window if any"

This reverts commit f5756b24.

ref #119
it is not really related to location window, actually, it does not work
as excpected: window numbers are not consistent:
- winnr() == 1
- vsplit, jump to new window
- winnr() == 1
- last window, winnr() == 2
- close winnr()==1, remaing window winnr()==1