This method is completely unnecessary as we have the readable Bearers
property already in place, so let's deprecate it.
Worth noting that the mm_modem_list_bearers() async/sync methods in
libmm-glib were not really using the ListBearers() method anyway, they
are using the property directly. These methods are NOT deprecated,
because they allow us to get a list of MMBearer objects, instead of
just the list of DBus paths that we get when reading the Bearers
property directly.
We also remove --list-bearers from the mmcli operation list as we have
already the same information in the standard modem output.
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