Remove obsolete platforms

As discussed at the PAC on 16th April, this MR removes the slc6 builds for 2017-patches and 2018-patches, since a centos7 build also exists, and removes the gcc49 build for 2017-patches, which had been kept for use on the grid on machines that do not support SSE 4.2 - after checking with the production team, this is no longer necessary, a very small number of such machines still exist and in any case DIRAC is protected against using these machines if the binary platform is not supported

@masmith @bcouturi @fredi FYI. When you set up the release and deployment for DaVinci/2017-patches later today or tomorrow, you should deploy only for "x86_64-centos7-gcc62-opt", "x86_64-centos7-gcc62-dbg" and make sure LbPlatform gives correctly the information that only centos7-gcc62 is supported