[assignGear] clear all unit traits by default

[NikkoJT]

Some unit types come with preconfigured traits. Sometimes these traits are not wanted. For example, GM special forces units have basically every trait regardless of their role.

assignGear should clear all traits to start with, so the unit ends up only having the traits granted by its assignGear class.

[SamLex]

Testing on dev (06ff60f) - PASS

• In non-dedicated MP with a mission created directly from the template.
  • Slotted an Engineer unit, class "r" - no traits, as expected
  • Slotted an Engineer unit, class "eng" - only Engineer/ExplosiveSpecialist traits, as expected
  • Slotted an Engineer unit, class "m" - only Medic trait, as expected
  • Slotted a Combat Life Saver unit, class "r" - no traits, as expected
  • Slotted a Combat Life Saver unit, class "eng" - only Engineer/ExplosiveSpecialist traits, as expected
  • Slotted a Combat Life Saver unit, class "m" - only Medic trait, as expected
  • Slotted an Ammo Bearer unit, class "r" - no traits, as expected
  • Slotted an Ammo Bearer unit, class "eng" - only Engineer/ExplosiveSpecialist traits, as expected
  • Slotted an Ammo Bearer unit, class "m" - only Medic trait, as expected