Remove the custom motd, and stop asserting a guess as the cause

Two problems, one of them mine twice over.

The motd is gone entirely. It had been wrong in a way that mattered — telling
anyone who logged in that onboard WiFi worked when it does not — and a login
banner is a bad place to keep claims that need revising every time we learn
something.

The worse issue is what it and the docs said about why the modem fails. I
inferred from the WOA-Project repository being named "Qualcomm-Reference-
Drivers" that its modem image was a generic reference build unsuited to this
machine's radio, and wrote that up as the explanation. That was a guess resting
on a repository name, and the naming is about redistribution rather than the
contents. Removed from both README and docs/firmware.md.

What is actually established stays: the modem boots, loads mpss, and dies at
"RF stuck in QLINK start state", roughly every 42 seconds. The cause is not
known. The machine has a SIM slot and its modem worked under Windows, so the
radio hardware is present — and the same cabinets supply the GPU, DSP and venus
firmware, all of which work, so whatever is wrong is specific to the modem.
The docs now say that and stop there.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XWRjNJMistCy6ngXH5aJLS

overlay/etc/motd.d/c630.in

@@ -1,11 +0,0 @@
-
-  Fedora @FEDORA_RELEASE@ for the @DEVICE_DESC@  (built @BUILD_DATE@ from @BUILD_REF@)
-
-  No per-model Qualcomm firmware ships in this image — it is not
-  redistributable. Supply it to get accelerated graphics, audio, the sensor
-  hub and hardware video decode: see docs/firmware.md, or run
-  `sudo c630-firmware` if this machine still has its Windows partition.
-
-  Onboard WiFi additionally needs modem firmware that no public source
-  provides. Use a USB dongle or tether a phone over USB.
-