git branch lists all local branches as well as
*, so the command you have given cannot actually be used to delete all merged branches and nothing else. Or if actually does do it while printing some error messages in the process, then I would never-the-less strongly advice against doing that.
Magit does not have a "delete all merged branches" command.
You can however list all branches merged into
C-u y - M o master RET (or
C-u y - m if
master is checked out). Then you can mark these branches using the region and delete them using