Remove `adios2cfg.xml`

[JamesMcClung]

While it in theory enables one to use different engines with the same executable, it leads to at least two pitfalls:

1. One must manually copy adios2cfg.xml into the run directory before each run
2. It is easy to forget to add a configuration to adios2cfg.xml, leading to bugs that have no basis in the code itself (see Fix adios2cfg #345)

It might be better if these configurations were encoded at compile-time.