Remove duplicate call to install() method

Resolves #528 

This removes a second call of `install()` that is already handled earlier by the `verify_database_present()` method.

Also removes duplicate use of the `wp_stream_no_tables` filter in `install()` that is also handled beforehand by `verify_database_present()`.

I've tested in single site and multisite. Fresh installs and DB upgrade routines are working in both.

@shadyvb @lukecarbis Please review