Refactoring overrides in midpoint()

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@28272 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Remove CmdCenter (redundant with CmdMidpoint)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@28273 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e