Remove bumpy mortar left of the wall
I know this is not the answer you are looking for, for this wall is ready for plastering as it is. It will need a scrubbing  or power washing to remove all or most of the dust left over from the original plaster.

To get the wall flat like you ask Will require at least scraping with a tool like this. (image courtesy Razorback)

This will be very time consuming and if the mortar is very strong, it may not do what you want. The most reliable way to flatten the wall is with a right angle grinder and a blade (disk) (Images courtesy Amazon) made for cleaning the surface. It will be a very, very, dusty job. You will need to wear a protective dust mask and face sheild.

The art of plastering will require a build up coat and a final layer, if you are planning on true plaster. There are newer versions of plaster look a likes out there, but most start with a base layer to prepare the wall for the finish coat. This eliminates the need for getting the wall perfectly flat.