Removed spannertools.DynamicTextSpanner.

Class was unused.

EXPLANATION: many versions ago Abjad spanners implemented a default LilyPond
grob which served as the target of overrides made to the spanner. The Abjad
DynamicTextSpanner managed overrides made to the LilyPond DynamicTextSpanner
grob. This stopped being the case a long time ago, however. You can now use
override() to override *any* LilyPond grob against *any* Abjad spanner or *any*
Abjad component. This means that the motivation for supporting an Abjad
DynamicTextSpanner class disappeared a long time ago.