There are two ways to delete a phase from your pipe. The first (and faster) is to click on the three dots in the far right side of the phase column and select the 'delete this phase' option. After you type 'confirm', your phase will be deleted.
The other way is accessing the phase settings. They can do so by clicking the three dots next to the phase's name and selecting the 'edit this phase' option or clicking on the gear icon on the upper right corner of the pipe then on 'phase settings'.
Once inside the phase list, click on top of the phase you want to delete and scroll down until you find the delete phase button. Click on it, type 'confirm' and your phase will be deleted.
Attention: If there are any cards in the phase you wish to delete, move them to another phase before deleting it, otherwise they’ll be deleted as well.