In 84728bd for CDRIVER-4363, mongoc_bulkwrite_set_session was declared with the BSON_EXPORT macro. This is inconsistent with the use of MONGOC_EXPORT in the corresponding header.

Per kevin.albertson@mongodb.com's comment in mongodb/mongo-c-driver#1806, there is no need for these macros in source files. We can additionally remove the occurrence in mongoc_client_encryption_new().