Revert "[gn build] (manually) port 6316129"

This reverts commit 5f4a334.
6316129 was reverted in 112aafc.

Author: nico

llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn

@@ -349,7 +349,6 @@ write_cmake_config("llvm-config") {
     "LLVM_NATIVE_TARGETINFO=1",
     "LLVM_NATIVE_TARGETMC=1",
     "LLVM_NATIVE_TARGETMCA=1",
-    "LLVM_UNREACHABLE_OPTIMIZE=1",
     "LLVM_USE_INTEL_JITEVENTS=",
     "LLVM_USE_OPROFILE=",
     "LLVM_USE_PERF=",