Don't call gp_upgrade_data() in gp_upgrade_db() on install

[ocean90]

If gp_upgrade_db() runs for the first time gp_upgrade_data() gets called with false for the $db_version parameter, thus the query for $db_version < 190 runs.

• Don't call gp_upgrade_data() if get_option( 'gp_db_version' ) returns false
• Remove the unused global import for $wpdb; from gp_upgrade_db()
• Remove the $db_version < 190 query since this will never run. The default version for all installs is 940.

[toolstack]

It will run in the case of a first install (false=0) but of course there will be no data to upgrade in that case.

Since it's not used should we just remove the entire gp_upgrade_data() function and call?

[ocean90]

No need to remove it if we're going to use it sooner or later. For example for #293.

[toolstack]

#293 won't need gp_upgrade_db() as it will probably just add columns to the database which will be handled by dbDelta().