drm/i915/gt: Delete the live_hearbeat_fast selftest

The test is trying to push the heartbeat frequency to the limit, which
might sometimes fail. Such a failure does not provide valuable
information, because it does not indicate that there is something
necessarily wrong with either the driver or the hardware.

Remove the test to prevent random, unnecessary failures from appearing
in CI.

Suggested-by: Chris Wilson <chris.p.wilson@intel.com>
Signed-off-by: Niemiec, Krzysztof <krzysztof.niemiec@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fe2vu5h7v7ooxbhwpbfsypxg5mjrnt56gc3cgrqpnhgrgce334@qfrv2skxrp47