Remove llvm-toolchain-15

[Jeremy Bícha]

Please remove llvm-toolchain-15 from questing. It has been removed from Debian in January. It was in main for Ubuntu 22.04 LTS but in universe after that.

$ reverse-depends -r plucky src:llvm-toolchain-15
Reverse-Recommends
==================
* libmkl-dev [amd64] (for libomp-15-dev)
* libmkl-rt [amd64] (for libomp-15-dev)

Reverse-Depends
===============
* libllvmspirvlib15 (for libllvm15t64)
* llvm-spirv-15 (for libllvm15t64)
* nvidia-cuda-toolkit [amd64 arm64 ppc64el]

nvidia-cuda-toolkit, libmkl-dev, and libmkl-rt are false positives since llvm-toolclhain-15 packages are only alternate dependencies.

$ reverse-depends -r plucky -b src:llvm-toolchain-15
Reverse-Testsuite-Triggers
==========================
* spirv-llvm-translator-15 (for clang-15)
* spirv-llvm-translator-15 (for llvm-15-dev)

Reverse-Build-Depends
=====================
* spirv-llvm-translator-15 (for clang-15)
* spirv-llvm-translator-15 (for libclang-15-dev)
* spirv-llvm-translator-15 (for llvm-15-dev)

$ reverse-depends -r plucky src:spirv-llvm-translator-15

$ reverse-depends -r plucky -b src:spirv-llvm-translator-15
Reverse-Build-Depends
=====================
* llvm-toolchain-15 (for llvm-spirv-15)