timeval: Remove time_disable_restart(), time_enable_restart() functions.

These functions will not have the same useful effect when Open vSwitch
becomes multithreaded, because time_disable_restart() will disable time
advancing for every thread, not just for the thread that calls it.

These functions are no longer used, so this commit removes them.

Signed-off-by: Ben Pfaff <blp@nicira.com>