Restore underline in LSP diagnostics

Using a better colorscheme makes the underlining more useful, as it
doesn't overrides syntax highlighting colors.  Therefore there is no
need to have it turned off.

nvim/language_server.vim

@@ -46,7 +46,6 @@ local on_attach = function(client, bufnr)
   vim.lsp.handlers["textDocument/publishDiagnostics"] = vim.lsp.with(
       vim.lsp.diagnostic.on_publish_diagnostics, {
         update_in_insert = true,
-        underline = false,
         virtual_text = false
       }
   )