Fix compilation warning by removing unused variable [NFC]

clang complained with

../tools/clang/tools/extra/clangd/FS.cpp:19:12: error: unused variable 'Err' [-Werror,-Wunused-variable]
  if (auto Err = FS.makeAbsolute(PathStore))
           ^
1 error generated.

llvm-svn: 343661