How does one drop a template database from PostgreSQL?

You can use the alter database command. Much simpler and safer than upsetting metadata.

postgres=# create database tempDB is_template true;
CREATE DATABASE
postgres=# drop database tempDB;
ERROR:  cannot drop a template database
postgres=# alter database tempDB is_template false;
ALTER DATABASE
postgres=# drop database tempDB;
DROP DATABASE
postgres=# 

Documentation

postgres=# UPDATE pg_database SET datistemplate='false' WHERE datname='template_postgis';
UPDATE 1
postgres=# DROP DATABASE template_postgis;
DROP DATABASE
postgres=#