test(lua/diagnostic_spec): remove unnecessary after_each()

There is already a call to clear() in before_each(), so after_each() isn't necessary.

test/functional/lua/diagnostic_spec.lua

@@ -79,10 +79,6 @@ describe('vim.diagnostic', function()
     ]])
   end)
 
-  after_each(function()
-    clear()
-  end)
-
   it('creates highlight groups', function()
     command('runtime plugin/diagnostic.vim')
     eq({