Remove unnecessary `onCommand` events from `package.json`

[gadenbuie]

https://github.com/posit-dev/pyshiny-vscode/blob/3635a936a35859af82b3a02bf8246142d0631635/package.json#L18-L24

The onCommand events are flagged by linters:

This activation event can be removed for extensions targeting engine version ^1.75 as VS Code will generate these automatically from your package.json contribution declarations.

I think this is safe: VS Code 1.75 was released in January 2023.