BusTransaction: remove confusing getter of connections

BusTransaction: remove confusing getter of connections

There is a DBusList* member of BusTransaction named "connections", while
its getter function bus_transaction_get_connections() returns
context->connections which in fact is a BusConnections pointer, this is
quite confusing. Because this is what bus_context_get_connections()
returns.

This patch call out to bus_context_get_connections() directly and remove
the then unused bus_transaction_get_connections().

https://bugs.freedesktop.org/show_bug.cgi?id=71597
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>