Remove conversion from string to DetectorSizeConstants in DetectorParams

We used to need to be able to accept a string as `DetectorSizeConstants` as it was a parameter from GDA but this is now used as a constant in Hyperion so we can remove the need to derive it from a string.

## Acceptance Criteria
* Remove ability to specify `DetectorSizeConstants` as a string
* `DetectorParams` still converts to json and back ok