Operation not supported (line 3)invoked from within "HTTP::header remove x-forwarded-for"

Couple of things. change your

while

to

if

and you should be good to go with the irule. However, if you are on v11.5+ you can use the HTTP profile to ignore the X-Forwarded-For header from clients by configuring X-Forwarded-For header acceptance to disabled. You can then also enable X-Forwarded-For header insertion in the profile to insert that header. This will be much more efficient than the iRule.