Simplify WP upgrade instructions

Removed a bunch of 4.6-era upgrade instructions that no longer apply.

@colemanw this generally looks good and it is time to simplify.

I would add a note that one should locate the settigs file and that there are two places that they should look <wordpress_root>/content-dir/uploads/civicrm/civicrm.settings.php and <wordpress_root>/wp-content/plugins/civicrm/civicrm.settings.php to locate this file. If it is in the latter location, then when they delete the plugin directory the settings file will be gone.

So perhaps we should say:

CiviCRM needs to use the civicrm.settings.php file during the upgrade. Please locate your civicrm.settings.php file. The default location is <wordpress_root>/content-dir/uploads/civicrm/civicrm.settings.php. If the file is there, please proceed. If not, the settings file may be in its legacy location <wordpress_root>/wp-content/plugins/civicrm/civicrm.settings.php. If you find it there, please move to <wordpress_root>/content-dir/uploads/civicrm/civicrm.settings.php

While this section is confusing, removing the section could lead to a failed upgrade if that file is in use.

Also, if you are using the <wordpress_root>/wp-content/plugins/civicrm/civicrm/settings_location.php file in your implementation, make a copy of this as well as you will need to restore it after upgrading.

For clearing cached files, I'd note:

Delete all files in your <wordpress_root>/<content-dir>/uploads/civicrm/templates_c directory and if it exists, all files in <wordpress_root>/<content-dir>/plugins/files/civicrm/templates_c

I may have some sites that started with CiviCRM 4.2 that still have /plugins/files/civicrm/templates_c in use.... That's my laziness, but it can't hurt to leave that in.

Thanks @kcristiano. Simplest workflow is probably for me to merge this & then you can do a followup MR with your additions.

mentioned in commit fa5c8aa0

merged

mentioned in merge request !372 (merged)