cvar to unbind both timer increment styles

Added an opt-in cvar to unbind both timer increment styles.

!960 (merged) which fixed an inconsistency between the timer increment styles (of from low to high being bound and high to low being unbound and showing seconds leading to the start of the match) made having an accurate timer for demoseeking and being able to vote as soon as g_start_delay ended very, very tedious. I would wish to revert this but preferably unbind both increment styles instead of having inconsistency.