Revert "Update docs around storing EJSON in MongoDB"

This reverts commit f91d8707787f70f045d10223370bba1137c56e80.

It turns out that while we did implement "storing EJSON in MongoDB", the
technique we chose is very flaky.  Some MongoDB commands (insert,
updates that don't increase the doc size?) let you store fields with
non-starting $, and others (update that does increase the doc size?)
don't.