Remove and cleanup text for adding to menu

There is a text called "If set to normal or tab, enter the text to use for the menu item.", it seems to give the idea that when you select either normal or tab you must enter a value here. But they are the only values available, as contrib adds options here the text might even not be valid anymore. Therefor I would suggest to streamline the descriptions here with that of Core menu link adding.

Before

After

I also removed something that wasn't RTL friendly.

Further improvements could include:
1) Use states, to display these fields "below" not next to the radio's so it doesn't create an often awkward small area to configure within.
2) Use more sensible labels than "normal", "tab" - as especially the "tabs" are confusing options - since they don't have to be themed that way.
3) Make the description into a textarea, just like Core.