Remove legacy repositories

[TotoGaz]

Merging of #81 and GEOS-DEV/GEOS#647 makes https://github.com/GEOSX/dockerBuilds and docker registry https://hub.docker.com/r/geosx/compiler useless.

They shall be deleted. Probably waiting a few days (maybe 1 or 2 weeks) to be sure that nothing is wrong would be wise.

https://hub.docker.com/r/geosx/ubuntu18.04-gcc7, https://hub.docker.com/r/geosx/centos7.5.1804-clang7.0.0 and https://hub.docker.com/r/geosx/centos7.5.1804-clang6.0.1 are made useless by GEOS-DEV/GEOS#808 too.

[rrsettgast]

@TotoGaz OK. We will delete them at the next documentation day.