core: drop check-uniq-files

Back a few years ago, when we were starting to think about top-level
parallel build, we were not sure how to deal with packages that
installed the same files, so we wanted to catch the situation to assess
how prevalent that was, before we decided what to do and how to address
it.

However, the trend nowadays is that packages will install in a
per-package target/ (and staging/ and host/), and the final directories
will be assembled in a reproducible (alphabetical) order, so if two
packages install the same file, the last one will win (as is currently
the case).

Besides, check-uniq-files reports loads of spurious errors when packages
get reinstalled (e.g. during development).

Finally, check-uniq-files is the only script called during the build,
that is written in python.

So, get rid of check-uniq-files.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>