rtw88: remove more references to rtw88fw

When I removed the Realtek rtww8x firmware from base I missed to
add rtw88fw.4 to ObsoleteFiles.inc [1].
While adding this, also remove a leftover reference in rtw88.4.

Reported by: Herbert J. Skuhra (herbert gojira.at) [1]
Fixes: cf94672363d6
MFC after: 3 days