Revert addition of waveform_mean_index for now

[rly]

Summary of changes

• Based on discussion today with @oruebel and @stephprince, we decided to revert the change to make "waveform_mean" and "waveform_sd" ragged because of confusing access patterns and conflict with the current 2D/3D schema. We will discuss this with the NWB TAB. The tentative plan is to create a new column "electrodes_waveform_means" and "electrodes_waveform_sds" and slowly phase out "waveform_mean" and "waveform_sd" in favor of this more general column.

Checklist

For all schema changes:

• Add release notes for the PR to docs/format/source/format_release_notes.rst.
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.
• Make sure that hdmf-common-schema points to the latest release and not the latest commit on the main branch.