Always use LineType.ChunkCombinedNan for lines in Bokeh

[ianthomas23]

Now that we have LineType.ChunkCombinedNan, always use it to render lines in BokehRenderer. Whatever lines are passed toBokehRenderer.lines are converted via convert_line_type and dechunk_lines to give a single NaN-separate points array which is used to create a Bokeh line glyph rather than the multi_line glyph used previously here.