remove "SQL Server" pyodbc driver name default

[Mike Bayer]

this is in mssql/pyodbc. it is only used for DSN-less connections, and should be removed; it apparently isn't the best "default", and if someone is specifying hostname and not DSN in any case, they need to set this up themselves.

[Mike Bayer]

• The hostname-based connection format for SQL Server when using pyodbc will no longer specify a default "driver name", and a warning is emitted if this is missing. The optimal driver name for SQL Server changes frequently and is per-platform, so hostname based connections need to specify this. DSN-based connections are preferred. fixes #3182

→ <<cset 4399431b53e5>>