Fix bug #2969: Double labels appear on empty lines of multi-line equations.

Fix bug #2969: Double labels appear on empty lines of multi-line equations.

This reverts part of r10553. There, an extra '//' was added when the last line was empty. However, this really makes an extra line and that is visible when the lines have a line number. The bug #2067 that would have been fixed by this does not seem to 'exist' anymore.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30795 a592a061-630c-0410-9148-cb99ea01b6c8