clock: Remove unnecessary #include <drivers/clock.hh>

Remove unused #include of <drivers/clock.hh>.
Except the clock drivers and <osv/clock.hh>, no source file now now
include this header. Rather, <osv/clock.hh> should be used. Code including
<sched.hh> will also get <osv/clock.hh> automatically.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>