Previously, the solver filtered out redundant backjumping messages twice, once
in 'Log.logToProgress' and again in 'Message.showMessages. However,
'showMessages' relied on the backjumping messages to determine where to insert
messages about missing packages. This led to missing "unknown package" messages
(part of issue #3617).

This commit removes the filtering in 'logToProgress', because it was redundant.