Documentation should not discuss flashing (proprietary) firmware for modem

Currently the "Mini-Tutorials" have a section on flashing modem firmware:

https://source.puri.sm/Librem5/developer.puri.sm/blob/f358d3afea8115411030947074c126909f5e619a/Boards/mini-tutorials.rst#L19

Firstly, we no longer use that Sierra Wireless modem. Second, providing instructions on how to flash proprietary firmware is a problem from the perspective of the FSF, both for the Librem 5's potential RYF certification and PureOS's listing as a GNU-FSDG satisfying distribution:

https://www.gnu.org/distros/free-system-distribution-guidelines.html#documentation

We should remove this section on the firmware flashing and remove mention of the MC7455 as this will just confuse people.