Remove Silo.patch once LLNL/Silo#550 is merged and released

[sbryngelson]

Background

MFC currently patches Silo at build time (toolchain/dependencies/Silo.patch) to change set(CMAKE_MODULE_PATH ...) to list(APPEND CMAKE_MODULE_PATH ...) in Silo's CMakeLists.txt. This is necessary because Silo's set() wipes out the -DCMAKE_MODULE_PATH=... MFC injects for Cray CCE (pointing to toolchain/cmake/cce/FindHDF5.cmake), causing SiloFindHDF5.cmake to fail to locate HDF5.

An upstream fix has been submitted: llnl/Silo#550

What to do when that PR is merged and a new Silo release is tagged

1. Bump GIT_TAG in toolchain/dependencies/CMakeLists.txt to the new Silo release tag.
2. Delete toolchain/dependencies/Silo.patch.
3. Remove the PATCH_COMMAND stanza from ExternalProject_Add(silo ...) in toolchain/dependencies/CMakeLists.txt.
4. Remove the Cray-conditional -DCMAKE_MODULE_PATH=... arg from the same ExternalProject_Add call (line 113). With list(APPEND) in Silo, the path set by MFC's parent cmake will be preserved automatically.
5. Verify the Silo build on Frontier (Cray CCE) still works.

This will fully resolve #621.