[NFC][clangd] Remove unused lambda capture

Avoid this warning:

llvm/clang-tools-extra/clangd/ClangdServer.cpp:365:23: warning: lambda
capture 'this' is not used [-Wunused-lambda-capture]

  auto Action = [Sel, this](decltype(CB) CB, std::string File,
                    ~~^~~~
1 warning generated.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@353760 91177308-0d34-0410-b5e6-96231b3b80d8