Remove duplicate Tests/irckitTests directory

[ForestKatsch]

Removes a duplicate unused directory in Tests that caused an error on macOS (who doesn't love case-insensitive filesystems?)

Cloning into 'IRCKit'...
remote: Enumerating objects: 752, done.
remote: Counting objects: 100% (136/136), done.
remote: Compressing objects: 100% (53/53), done.
remote: Total 752 (delta 68), reused 130 (delta 65), pack-reused 616 (from 1)
Receiving objects: 100% (752/752), 141.76 KiB | 1.23 MiB/s, done.
Resolving deltas: 100% (478/478), done.
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'Tests/IRCKitTests/irckitTests.swift'
  'Tests/irckitTests/irckitTests.swift'
  'Tests/IRCKitTests/XCTestManifests.swift'
  'Tests/irckitTests/XCTestManifests.swift'

There don't seem to be any tests anyway so this shouldn't have any impact.