Remove HDF5 Build Hacks

[henryleberre]

While CMake ships its own FindHDF5, we had to create our own toolchain/cmake/FindHDF5.cmake to accommodate non-cooperative systems and compilers (afaik only Cray CCE). This issue arises again when we attempt to build the LLNL Silo dependency because it uses the built-in (CMake-provided) FindHDF5. To work around this, we patch Silo to allow us to inject our toolchain/cmake/FindHDF5.cmake. This is terrible. See:

https://github.com/MFlowCode/MFC/blob/63c79cbf041c71985b4123aa9652a7df83604b44/toolchain/dependencies/Silo.patch#L10-L19

https://github.com/MFlowCode/MFC/blob/63c79cbf041c71985b4123aa9652a7df83604b44/toolchain/dependencies/CMakeLists.txt#L80

As a result of this, HDF5 is the last of our dependencies that we always build even when a compatible version is present on the system. This is also why the --sys-hdf5 command-line argument is still supported by ./mfc.sh but --sys-fftw and --sys-silo are no longer accepted.

Good luck to whoever is tasked with resolving this in the future. I do not think this will ever be fixed but I would like to be proven wrong.