pixelstore option removed; documentation is obsolete

[jdswinbank]

Way back in 2016 TraP gained the ability to store images in the main (Postgres) database, and the old MongoDB “pixel store” was removed. The pixelstore option on setup was zapped in the process.

However, six years later, the documentation still suggests using pixelstore (despite the fact that pip itself actually complains when you specify it!) and spouts untruths about the use of MongoDB. Let's fix it.

[AntoniaR]

We should make sure this is fully removed in R7.