Vim: remove `jj` mapping

Some people I code with now prefer to use `for` loop variables like
this:

  for (var ii = 0; ii < thing.max; ii++) {
    for (var jj = 0; jj < other.max; jj++) {
      for (var kk = 0; kk < that.max; kk++) {
        // ...
      }
    }
  }

Having to type `jj` in insert mode like that is painful in such a
codebase, so remove the mapping. I've got something much better now
anyway (dual-purposing Caps Lock as Esc and Ctrl).