Removing `llvm-tools` from build dependencies in `bdt create` is undesirable

This was done to overcome another issue regarding llvm-tools and its dependence to libllvm10.

One of the (build) packages we depend on wants to have libllvm10=10.0.0, but "llvm-tools" requires libllvm10=10.0.1. This creates a conflict. Build requirements from our packages will end-up bringing in the requirement for "llvm-tools".

To test this, on a macOS system, try the following:

$ conda create --dry-run --name xxx --override-channels --channel=http://www.idiap.ch/software/bob/conda/label/beta --channel=http://www.idiap.ch/software/bob/conda --channel=defaults --dry-run 'bob-devel=2021.01.28.*' clang llvm-tools

The problem is, possibly, that our own bob-devel brings in dependencies that make the above build tools conflict. Removing any of the 3 packages listed above from the command-line makes the command-line work again. Once that command (or a variant with a newer version of bob-devel) works, the problem is fixed and a patch is no longer required.