pkg/semtech-loramac: don't force ztimer rtt backend

[aabadie]

Contribution description

This PR removes the hard dependency of ztimer RTT backend used by semtech-loramac package. AFAIU, with #16553, the RTT backend is only used when the periph_rtt is provided by the board.

Testing procedure

• Green Murdock
• Semtech-loramac package based applications are still functional
• Check that ztimer RTT backend is used with b-l072z-lrwan1 board (which is the default for semtech-loramac applications)
• Check that examples/lorawan and tests/pkg_semtech-loramac can be built with im880b (no RTT provided so cannot be used in current master)

Issues/PRs references

Follow-up of #16553

[jia200x]

Check that ztimer RTT backend is used with b-l072z-lrwan1 board (which is the default for semtech-loramac applications)

BOARD=b-l072z-lrwan1 make info-modules | grep ztimer_periph_rtt 
ztimer_periph_rtt

[jia200x]

Check that examples/lorawan and tests/pkg_semtech-loramac can be built with im880b (no RTT provided so cannot be used in current master)

RIOT/examples/lorawan/ BOARD=im880b make > /dev/null && echo "OK"              
OK

RIOT/tests/pkg_semtech-loramac/ BOARD=im880b make > /dev/null && echo "OK"
OK

[jia200x]

Tested on nucleo-l452re:

2021-06-17 14:40:35,526 # Connect to serial port /dev/ttyACM0
Welcome to pyterm!
Type '/exit' to exit.
loramac set deveui ...
2021-06-17 14:42:30,028 # loramac set deveui ...
> loramac set appeui ...
2021-06-17 14:42:39,155 # loramac set appeui ...
> loramac set appkey ...
2021-06-17 14:42:47,962 # loramac set appkey ...
> loramac join otaa
2021-06-17 14:42:51,158 # loramac join otaa
2021-06-17 14:43:00,525 # Join procedure succeeded!
> loramac tx test
2021-06-17 14:43:12,351 # loramac tx test
2021-06-17 14:43:15,824 # Received ACK from network
2021-06-17 14:43:15,826 # Message sent with success

[jia200x]

ACK