Remove ability to turn off Gretl

There is a CMake option called `SMITH_ENABLE_GRETL` that implies you can turn off Gretl, which you cannot do. It should be removed.

Search the CMake configuration for the SMITH_ENABLE_GRETL option and trace its references. Remove the option and any associated handling, then run the project’s CMake configuration to confirm it no longer advertises a switch for disabling Gretl.

Written by the indexing model from the issue text.