improve warmup handling, allow it to be disabled

[Lidbetter]

• remove all warmup related commands out of the default game config
• skip any warmup related tasks by setting sm_retakes_warmuptime 0
• prevent clamp minimum warmup time to 6 seconds (preventing infinite warmup end spam/failed rounds restarts)

[crashzk]

@Lidbetter could you tell me more about this warmup bug when set to less than 6 seconds, infinite warmup spam and such.

I ask because my server is configured with the following warmup configuration and I never noticed this bug.

mp_do_warmup_period 1
mp_warmup_pausetimer 0

[Lidbetter]

@crashzk set: mp_warmuptime 4 the server will infinitely attempt to restart the round to end the warmup