Finish removing ENCAD shifts and table generation

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