avoid storing line numbers in the baseline file

from my understanding the line number is not used when matching errors against the baseline file, so it doesn't need to be stored.

in my experience, storing the line number results in very noisy diffs when updating the baseline file, and also means it needs to be updated much more often, since most changes to a file would result in most of the line numbers being changed (which would be annoying when #4449 is done)