WordPress.org

1. Download the plugin zip file & unzip it
2. Copy the folder always-edit-in-html into the directory /wp-content/plugins/
3. Activate the plugin through the 'Plugins' menu in WordPress

Open up a page or post and you will now see the "Always Edit In HTML" option box on the right column.

Check the option and save.

Next time you open the same page or post it will open up in HTML mode, removing the "Visual" and "HTML" tabs.

Uncheck the option at any time to get the "Visual" and "HTML" buttons and default opening behaviour back.