patch - Remove PPP round 2

[Peter Naulls]

I revised my earlier removal of PPP from the ModemManager build to be more 
complete. I'd like to remove it from the enums, but these are generated by meson
and it'll still pick up #ifdefed out items - I couldn't find a mechanism to make 
this work.

Also a patch against the Makefile in OpenWrt to use this and update from 1.20.2
to 1.20.4. Also note the previous patch I posted with the handling of .conf 
files vs no MBIM.

To be clear on this, even on largely well behaved modems, timeouts can still 
occur, or a reset can happen at the "wrong" time, or other race conditions, 
causing a trigger into PPP mode when it is not required. For my use, attempting
to use PPP is never the answer - we need to fall back to an error condition, or
retry etc - cycling on trying to use PPP results in grief, from painful and 
extensive experience of this.
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