Remove `read` from `JobDatabaseInterface`

[soxofaan]

spin off from #619 (comment)

The read method is not useful anymore in the general JobDatabaseInterface interface.
At the moment it is only used "from the outside" at

openeo-python-client/openeo/extra/job_management.py

Line 367 in 47172cd

df = job_db.read()

where the result is actually unused.

So for the sake for development of more advanced job dbs like in #619 I think it's best to eliminate this part of the API

[soxofaan]

the read method can just be moved down to FullDataFrameJobDatabase apparently