Removes two unused vars of the gamemode datums, uplink_welcome and uplink_uses.

uplink_welcome was actually used as a greeting text from uplinks, now they'll always show "Syndicate Uplink Console:". The removed strings weren't nothing important, actually, with the addition of the uplinks in cargo, you were able to metagame the ongoing gamemode.
Merge pull request #4786 from Aranclanos/gamemodeunusedvars

Removes two unused vars of the gamemode datums.