doc: update version in doc/mysql.txt; remove bdb support for mysql.

Remove obsolete info from doc and code.

doc/mysql.txt

@@ -13,11 +13,8 @@ Prerequisites
 To use MySQL as the backend for storing roundup data, you also need 
 to install:
 
-1. MySQL RDBMS 4.0.18 or higher - https://www.mysql.com/. Your MySQL
-   installation MUST support InnoDB tables (or Berkeley DB (BDB) tables
-   if you have no other choice). If you're running < 4.0.18 (but not <4.0)
-   then you'll need to use BDB to pass all unit tests. Edit the
-   ``roundup/backends/back_mysql.py`` file to enable DBD instead of InnoDB.
+1. MySQL RDBMS 8.0.11 or higher - https://www.mysql.com/. Your MySQL
+   installation MUST support InnoDB tables.
 2. Python MySQL interface - https://pypi.org/project/mysqlclient/
 
 

roundup/backends/back_mysql.py

@@ -145,10 +145,7 @@ class Database(rdbms_common.Database):
     implements_intersect = 0
 
     # Backend for MySQL to use.
-    # InnoDB is faster, but if you're running <4.0.16 then you'll need to
-    # use BDB to pass all unit tests.
     mysql_backend = 'InnoDB'
-    # mysql_backend = 'BDB'
 
     hyperdb_to_sql_datatypes = {
         hyperdb.String:   'TEXT',