Change edgeloop delete to use dissolve, fixes bug [#35738].

Was using edge-slide & remove-doubles but this was error prone since remove doubles could fail in some cases or find doubles where it shouldn't (with very small scale objects).

This gives more predictable behavior when the edges of a loop wouldnt slide (in that case they would just drag over to one of the sides with no user control)
and multiple edge loops work better too. eg:
- http://www.graphicall.org/ftp/ideasman42/edge_loop_del_update.png


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57445 954f8c5b-7b00-dc11-b283-0030488c597c