repos.yaml: Use our mirror for mer-core

Unfortunately Mer doesn't create the repository data for x86_64,
the bug #763 was open last year and nobody actually fixed it even
if I asked at least 3 times on IRC.

I'm now forced to mirror mer-core myself to enable the x86_64 build.

Bug report: https://bugs.merproject.org/show_bug.cgi?id=763
Remove mer-tools repository

It doesn't have x86_64 packages hence I'm branching or redoing the
packages we need for Maui in the maui OBS project.