knife tool: Clean up line hits when starting a new cut

Fixes intersection points of the previous cut still being drawn when
starting a new cut.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61203 954f8c5b-7b00-dc11-b283-0030488c597c
knife tool: Remove unused 'extend' variable

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61204 954f8c5b-7b00-dc11-b283-0030488c597c