[clang] Remove workaround for old LLVM_ENABLE_PROJECTS=libcxx build

[ldionne]

Summary

We don't support the LLVM_ENABLE_PROJECTS=libcxx build anymore (and have
not for over a year), so it should be fine to remove this workaround now.

[ldionne]

CCing some folks that were watching D132324.

[phosek]

LGTM

[MaskRay]

Related: D122444

clang/lib/Driver/ToolChains/Linux.cpp

#endif
   if (IsRISCV) {
     StringRef ABIName = tools::riscv::getRISCVABI(Args, Triple);
     addPathIfExists(D, concat(SysRoot, "/", OSLibDir, ABIName), Paths);
     addPathIfExists(D, concat(SysRoot, "/usr", OSLibDir, ABIName), Paths);
   }
 
   Generic_GCC::AddMultiarchPaths(D, SysRoot, OSLibDir, Paths);
 
-  // The deprecated -DLLVM_ENABLE_PROJECTS=libcxx configuration installs
-  // libc++.so in D.Dir+"/../lib/". Detect this path.
-  // TODO Remove once LLVM_ENABLE_PROJECTS=libcxx is unsupported.
-  if (StringRef(D.Dir).startswith(SysRoot) &&
-      D.getVFS().exists(D.Dir + "/../lib/libc++.so"))
-    addPathIfExists(D, D.Dir + "/../lib", Paths);
-
   addPathIfExists(D, concat(SysRoot, "/lib"), Paths);
   addPathIfExists(D, concat(SysRoot, "/usr/lib"), Paths);
 }
 
 ToolChain::RuntimeLibType Linux::GetDefaultRuntimeLibType() const {
   if (getTriple().isAndroid())
     return ToolChain::RLT_CompilerRT;
   return Generic_ELF::GetDefaultRuntimeLibType();