Remove database config for PostgreSQL and MySQL

Removed PostgreSQL and MySQL configuration settings from the example environment file.

src/.env.example

@@ -20,27 +20,6 @@ db_type=sqlite
 sqlite_database_filename=
 sqlite_table_prefix=
 
-# postgres config
-postgres_host=127.0.0.1
-postgres_port=3306
-postgres_user=mysql_user
-postgres_passwd=mysql_password
-postgres_database=database_name
-postgres_table_prefix=
-postgres_max_idle_conns=10
-postgres_max_open_conns=100
-postgres_max_life_time=6
-
-# mysql config
-mysql_host=127.0.0.1
-mysql_port=3306
-mysql_user=mysql_user
-mysql_passwd=mysql_password
-mysql_database=database_name
-mysql_table_prefix=
-mysql_max_idle_conns=10
-mysql_max_open_conns=100
-mysql_max_life_time=6
 
 # sqlite runtime store config
 runtime_sqlite_filename=epusdt-runtime.db