Re-do the mesa libelf entanglement without a split.

Drop the separation of the packages, instead make sure libelf isn’t used:

• r600 with llvm requires libelf
• amdvk requires libelf
• radeonsi requires libelf
• gallium-opencl requires libelf

This commit disables all of the above and also drops libelf-dev build dependency. We also declare Build-Conflicts against it to prevent accidentally linking to it.

libgl1-mesa-dri-llvm is now a transitional package also provided by libgl1-mesa-dri. It shouldn’t be depended on since it will be removed in future.