How to remove this flow restrictor in hansgrohe hand shower?
To collect the comments into an answer:
Use a small piece of metal to lift/bend each of the metal prongs up so it's no longer pressing into the plastic on the outside, holding the flow restrictor in place. Consider using one or more of the following to do so:

A small dental-type pick*
A small screw driver
A hefty paperclip
A sturdy piece of wire bent into a hook

You might have to push the restrictor slightly down while pulling up on the tab to release the end of the tab from where it's seated into the plastic. Simply pulling up may actually work against you to further embed the tab into the plastic wall.
It appears that there are 6 tabs holding it in place, once you get 3 of them bent, there will likely be enough play to get it out without having to bend the rest of them.
If you're finding it difficult to get the tabs to bend, you may consider leaving the restrictor in place and simply drilling out the plastic center, thus reducing the restriction. If you do this, be sure to thoroughly rinse out nozzle before reinstalling it. You want to be sure to get all the plastic swarf out the large end instead of letting it clog up the shower head.
*A pick like one of these:
 
Images courtesy of Harbor Freight, where the items depicted are available for under $5 US. No particular recommendation made, though they are a good source of cheap stuff. Neither an employee, owner, nor stock holder, but I'm always thrilled when my purchases last for a 2nd use!