Remove selector=None keywords from abjad.spanners.* functions

[trevorbaca]

OLD (before Abjad 3.20):

    abjad.spanners.beam(argument, selector=abjad.select.leaves)
    abjad.spanners.glissando(argument, selector=abjad.select.leaves)
    abjad.spanners.hairpin(argument, selector=abjad.select.leaves)
    abjad.spanners.horizontal_bracket(argument, selector=abjad.select.leaves)
    abjad.spanners.ottava(argument, selector=abjad.select.leaves)
    abjad.spanners.phrasing_slur(argument, selector=abjad.select.leaves)
    abjad.spanners.piano_pedal(argument, selector=abjad.select.leaves)
    abjad.spanners.slur(argument, selector=abjad.select.leaves)
    abjad.spanners.text_spanner(argument, selector=abjad.select.leaves)
    abjad.spanners.tie(argument, selector=abjad.select.leaves)
    abjad.spanners.trill_spanner(argument, selector=abjad.select.leaves)

NEW (beginning with Abjad 3.20):

    leaves = abjad.select.leaves(argument)

    abjad.spanners.beam(leaves)
    abjad.spanners.glissando(leaves)
    abjad.spanners.hairpin(leaves)
    abjad.spanners.horizontal_bracket(leaves)
    abjad.spanners.ottava(leaves)
    abjad.spanners.phrasing_slur(leaves)
    abjad.spanners.piano_pedal(leaves)
    abjad.spanners.slur(leaves)
    abjad.spanners.text_spanner(leaves)
    abjad.spanners.tie(leaves)
    abjad.spanners.trill_spanner(leaves)