extension: Remove `tooltip_text` from `SlashCommandManifestEntry` (#16306)

This PR removes the `tooltip_text` field from
`SlashCommandManifestEntry`s.

The `tooltip_text` is currently only used to set the `menu_text` on a
slash command, which is only used for featured slash commands.

Since slash commands from extensions are not currently able to be
featured, we don't need extension authors to provide this field in the
manifest.

This is a backwards-compatible change.

Release Notes:

- N/A