[runtimes][NFC] Remove dead code for Standalone builds

Standalone builds have been deprecated and then removed for a while now.
Trying to use standalone builds leads to a fatal CMake error, so this
code is all dead. Remove it to clean things up.

Differential Revision: https://reviews.llvm.org/D125561

GitOrigin-RevId: 0a22dfcb11c05cbd4f654c8ef1868a4bc6085140