remove old CE-related docs from handbook

Created by: sqs

I asked about these in our CE team meeting on 2021-01-26 and got confirmation we could remove these.

Some of the deleted files in this commit were actually moved to gdocs:

• QBR guide moved to gdoc for easier live editing: https://docs.google.com/document/d/1gFRn2SkX19sU0GSMGndNkk-I9cFe7FlN3xlZ2UX3Frs/edit#heading=h.eoe1yhgq0z1j
• Sourcegraph 101/102 moved to gdoc for easier live editing: https://docs.google.com/document/d/1nFePrSIcIakMmjOEY01vNc6VRe7WiJ0iOWygeZlbpYw/edit#heading=h.dwjpi5sjkp53
• 20-min comprehensive demo script moved to gdoc for easier live editing: https://docs.google.com/document/d/1q903Yl-vkOqkQ4e3JRiw-u8x8aJ50iTezllzcj_MJWc/edit#heading=h.ahfx9g5c1qmy

The other deleted stuff was old and obsolete documentation.