shared/efi-loader: remove check that uses absolute tick value

sd-boot uses rdtsc to set those timestamps. There is no guarantee that the tsc
has any particular absolute value.

On my VM:
$ head /sys/firmware/efi/efivars/LoaderTime*
==> /sys/firmware/efi/efivars/LoaderTimeExecUSec-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f <==
4397904074
==> /sys/firmware/efi/efivars/LoaderTimeInitUSec-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f <==
4396386839
==> /sys/firmware/efi/efivars/LoaderTimeMenuUSec-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f <==
4396392521

$ build/test-boot-timestamps
...
LoaderTimeExecUSec=4396386839 too large, refusing.
Failed to read EFI loader data: Input/output error
Assertion 'q >= 0' failed at src/test/test-boot-timestamps.c:84, function main(). Aborting.

(with patch)
$ build/test-boot-timestamps
...
EFI Loader: start=1h 13min 16.386s exit=1h 13min 17.904s duration=1.517s
Firmware began 1h 13min 17.904074s before kernel.
Loader began 1.517235s before kernel.
Firmware began Tue 2020-05-26 11:04:13 CEST.
Loader began Tue 2020-05-26 12:17:30 CEST.
Kernel began Tue 2020-05-26 12:17:31 CEST.