Original ticket description:

Currently the bson_serialization_type for decimal in basic_types.idl is double. 

We think that this is likely just a copy-paste error from the "double" type defined immediately above in the basic_types.idl file, and, when we tested simply changing the bson_serialization_type of decimal to "decimal," we did not notice any failures.

However, we definitely didn't test this exhaustively, so we've left the type as is ("double") for now, in the interest of not conflating any errors that may arise from this with other pull requests.

New ticket description:

Remove decimal type from basic_types.idl because the type is not used.