Remove support for UMESIMD backend

Discussed initially in the biweekly meeting of the 25th October. The UMESIMD package providing an optional SIMD backend to VecCore and hence VecGeom is no longer maintained and is known to have issues in testing with VecCore. VecCore and eventually native compiler, support for std::simd will render it obsolete.

This MR is pretty simple and removes UMESIMD as an allowable value to VecGeom's BACKEND CMake option. Other uses/references to UMESIMD are then removed in C++/CMake/CI code. Downstream clients shouldn't be relying on a particular backend, but should be double checked.

@amadio also noted that we may want to use this to check and cleanup code that is using obsolete headers such as backend/vc/Backend.h in ABBoxBenchmark.cpp. I haven't done an exhaustive search yet, but the changes here do not cause significant breakage at least on local builds.