MDL-42926 Bootstrap base: Remove 6px spacing between form elements

This was accidentally copied from the css for base. It is only visible when
the form fields are displayed with the label above the field. (less than 1200px).
Merge branch 'MDL-42926-master' of git://github.com/damyon/moodle
Moodle release 2.6
weekly back-to-dev release 2.7dev