When ifcfg-rh writer persists a connection profile to disk, it would
first load the pre-existing file, modify it, and write it back.

In particular, it aims to keep comments and newlines, the order of
variables, and unknown variables.

Until recently, when writing the profile NetworkManager would set the
keys that make up the new profile, and explicitly clear some of the
keys. Since [1] (1.23.1), NetworkManager will no longer explicitly clear
any keys, instead, all (well known) keys that were not explicitly
set will be cleared automatically.

In this case, there is a change in behavior because the variable
BOOTPROTO would no longer be preserved. Just don't check for that.

You can see that previously the variable was unset, because it kept
the single quotes BOOTPROTO='static'. Had NetworkManager touched the
variable, it would have removed these quotes. But this variable was
really bogus, it has no place in the profile as it is. It's good that
we clear it now.

Note that entirely unknown variables would still be preserved.

[1] NetworkManager@07262b16