Remove backend options from the Everest configuration

[verveerpj]

Is your feature request related to a problem? Please describe.
In the Everest configuration, both the optimization and samplers sections have a backend option. This is used to specify which plugin/backend should be used to look for algorithms/methods that user specifies with the algorithm or method keys. The backend is not strictly necessary anymore, since ropt will search through all its available backends to find an algorithm/method. Further, if desired, the backend can be specified by prefixing the algorithm/method keyword like this: scipy/slsqp. Therefore this keyword can be removed.

Describe the solution you'd like
The keyword should be deprecated and eventually removed. The documentation should clarify how algorithms/methods are found.

Describe alternatives you've considered
N/A

Additional context
Implementation should be straightforward, since ropt already uses this approach.