obslog: remove now-unused code for plain styling We now always use a non-plain formatter for the non-local-filtering code paths, so we can remove the code for plain styling. I couldn't figure out how to replace the fm.formatlist() by something less about formatting, so I've only removed the irrelevant arguments from those calls.

obslog: redefine default output as a template, unless -f was given This was easier than I expected to do without affecting any test output at all. `hg obslog -f` is pretty broken with templating. For example, it inserts a single "date" field, but it provides two values for it if the dates are different. That results in a traceback when running e.g. `hg obslog -f -Tfoo` with non-local obsmarkers with different dates. We need to figure out what we want to put in the template for the filtered case. I've just left it out for now.