Removes references to the Syndicate and renames nuclear emergency mode.

Testing this locally is a bitch. I've done some basic stuff, checking that it compiles/starts, etc. Downstream probably wants to avoid this PR, as it's a solely RP-driven change and may lead to player confusion.

See also: http://baystation12.net/forums/viewtopic.php?f=5&t=11461&view=unread#unread
- Renames nuke mode to 'mercenary'.
- Replaces 'nuke op' with 'merc'.
- Replaces 'syndicate' and 'syndicate agent' with 'mercenary', 'merc', 'operative', etc.
- Removes the nuke flavour text for nuke mode.
- Doesn't mess much with nuke objectives.
- Changes the autotraitor welcome message and some objective text.
- Removes some redundant vars from several game modes.
- Doesn't touch the syndicate beacon.
- Doesn't touch the syndicate elite shuttle file (what even is that?).
- Doesn't touch Syndicate bans.
- Doesn't modify vars or paths.
- Doesn't modify away mission area names.
- Doesn't mess with faction fluff text.
- Doesn't touch the manuals.
- Doesn't touch corpses or NPCs.
- Left the Syndicate Bomb in as a callback, it's harmless.