fix: hide axis ticks from aria since they are just adding noise

[domoritz]

Fixes #2019

• Hide axis ticks since they just add noise (VoiceOver just exposes them as "image")
• Keep axis labels as the default. In the future, it would be great if axes actually described the scale range (as done by default in Vega-Lite). e.g. "0% to 13%".

I tested this with http://localhost:8008/?test=letterFrequencyBar.