Clear the DiagnosticDeprecated group

I don't find this terribly useful, and NeoVim's built-in LSP completion
popup uses this for applying highlights to suggested items, which is
annoying.

colors/grey.lua

@@ -271,6 +271,7 @@ local highlights = {
   DiagnosticHint = { fg = grey, bold = true },
   DiagnosticInfo = { fg = blue, bold = true },
   DiagnosticWarn = { fg = dark_yellow, bold = true },
+  DiagnosticDeprecated = {},
   -- Make
   makeTarget = { link = 'Function' },
   -- Markdown