python api: add functionality to remove vertex color layers.
note: that this intentionally removes check to exit vpaint mode when a vertex color layer is removed,
	  since being in vertex-paint mode without a vertex color layer is supported.

also minor change to drawing camera limits while picking from previous commit.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55480 954f8c5b-7b00-dc11-b283-0030488c597c