Remove Discord URL placeholder from translations (#4645)

## Description:

This value was a sample URL used as an input placeholder, and it was
removed because translating it would be meaningless and could
potentially alter the URL format incorrectly.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory

## Please put your Discord username so you can be contacted if a bug or
regression is found:

aotumuri

resources/lang/en.json

@@ -206,7 +206,6 @@
     "discord_section_title": "Discord",
     "discord_url_hint": "Use an invite set to never expire.",
     "discord_url_label": "Discord Invite URL",
-    "discord_url_placeholder": "https://discord.gg/...",
     "error_already_member": "Already a member",
     "error_banned": "You are banned from this clan.",
     "error_banned_reason": "You are banned from this clan. Reason: {reason}",