tcp: remove CTLFLAG_NEEDGIANT from sysctl(9) handlers related to timers

They all just modify a global word sized variable via
sysctl_msec_to_ticks(), which is wrapper around sysctl_handle_int(). Note,
they all were marked with CTLFLAG_NEEDGIANT in 7029da5c36f2 merely because
it was not obvious whether they are mpsafe or not.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D50890