The new Gmail has undergone a huge refresh. The Web version now has a tabbed inbox to support categories and the mobile apps have an updated interface as well. If you've updated to the new Android app, you may have noticed that there's no longer a trash button in the toolbar when viewing a message. The new toolbar only has buttons for archive, mark, and move. If you flip the screen orientation to landscape, you'll also see an additional button for labels.
If you prefer to delete certain messages, rather than archiving everything, you can still use the trash button to quickly delete messages without having to go to the menu first. You just need to enable it in settings. Here's how:
Go to Settings > General settings, then tap on Archive & delete actions. You'll see a pop-up with three options: Show archive only, Show delete only, and Show archive & delete. Tap on one of the actions that include "delete" to enable the trash button.
That's it. The next time you open a message, you'll see the familiar trash button in the toolbar.