Revert "[clangd] Fall back to selecting token-before-cursor if token-after-cursor fails."

This reverts commit b60896fa.
Breaks building with gcc:

/usr/include/c++/7/bits/stl_construct.h:75:7: error: use of deleted function ‘clang::clangd::Tweak::Selection::Selection(const clang::clangd::Tweak::Selection&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /home/buildslave/buildslave/clang-cmake-armv7-selfhost-neon/llvm/clang-tools-extra/clangd/ClangdServer.h:28:0,
                 from /home/buildslave/buildslave/clang-cmake-armv7-selfhost-neon/llvm/clang-tools-extra/clangd/ClangdServer.cpp:9:
/home/buildslave/buildslave/clang-cmake-armv7-selfhost-neon/llvm/clang-tools-extra/clangd/refactor/Tweak.h:49:10: note: ‘clang::clangd::Tweak::Selection::Selection(const clang::clangd::Tweak::Selection&)’ is implicitly deleted because the default definition would be ill-formed:
   struct Selection {
          ^~~~~~~~~
/home/buildslave/buildslave/clang-cmake-armv7-selfhost-neon/llvm/clang-tools-extra/clangd/refactor/Tweak.h:49:10: error: use of deleted function ‘clang::clangd::SelectionTree::SelectionTree(const clang::clangd::SelectionTree&)’
In file included from /home/buildslave/buildslave/clang-cmake-armv7-selfhost-neon/llvm/clang-tools-extra/clangd/refactor/Tweak.h:25:0,
                 from /home/buildslave/buildslave/clang-cmake-armv7-selfhost-neon/llvm/clang-tools-extra/clangd/ClangdServer.h:28,
                 from /home/buildslave/buildslave/clang-cmake-armv7-selfhost-neon/llvm/clang-tools-extra/clangd/ClangdServer.cpp:9:
/home/buildslave/buildslave/clang-cmake-armv7-selfhost-neon/llvm/clang-tools-extra/clangd/Selection.h:96:3: note: declared here
   SelectionTree(const SelectionTree &) = delete;
   ^~~~~~~~~~~~~

e.g. here:
http://lab.llvm.org:8011/builders/clang-cmake-armv7-selfhost-neon/builds/2714
http://lab.llvm.org:8011/builders/clang-ppc64be-linux/builds/41866