Revert "Adding support for overriding LLVM_ENABLE_RUNTIMES for runtimes builds."
Revert "Adding support for overriding LLVM_ENABLE_RUNTIMES for runtimes builds."

This reverts commit r371566: this doesn't correctly handle the case
where LLVM_ENABLE_RUNTIMES contains a list of values.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371579 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm