Finish removing ENCAD shifts and table generation

The force field was removed in 2014, and the functional forms can't be used any more since the group scheme is removed. The table-generation functions were also broken in several ways. Anybody wanting to use this force field in the recent past (or perhaps future) needs to use user tables.

The tpr-facing enums have their numerical values preserved, along with changes to indicate that the symbol values are unused. This means that old tpr files can still be read correctly, while maintainers don't have extra work to understand what is going on. The internal code can be fully removed.

Fixes #3477 (closed)