device-nokia-n900: enable use of rtc time at boot

In https://gitlab.com/postmarketOS/pmaports/-/merge_requests/1166, swclock was erroneously activated for Nokia N900. The consequence has been that the device could not use time from the rtc. This is a partial revert of that commit.

There is also no need to specify rtc device manually in /etc/conf.d/hwclock as the rtc device is correctly set kernel side, https://gitlab.com/postmarketOS/pmaports/-/blob/master/device/community/linux-nokia-n900/config-nokia-n900.armv7#L4407-L4409

Additional info: It is true that the rtc backup battery is dead on most Nokia N900 units, leading to time being lost if the main battery is removed for a short while. However, this means time is wrong only after battery removal, and once set via NTP (or manually), it is correct on every boot. With swclock, it is wrong on every single boot until set.

Related issue: https://gitlab.com/postmarketOS/pmaports/-/issues/398#note_304861329 - including N900 in the list was a mistake, possibly caused by the rtc backup battery issue mentioned above.

Other: swclock does not work properly in pmOS? I did not spend a lot of time troubleshooting it because focus was on the rtc, but it seems that the shutdowntime mentioned at https://gitlab.com/postmarketOS/pmaports/-/issues/398#note_331455114 was not getting created/touched, at least on my device, hence time was wrong on every boot as already mentioned. Perhaps a separate issue for testing that could be opened