Tip from here

edit squid.conf

vi /etc/squid/squid.conf

Find the following section of squid.conf and add forwarded_for off

#  TAG: forwarded_for   on|off
#       If set, Squid will include your system's IP address or name
#       in the HTTP requests it forwards.  By default it looks like
#       this:
#
#               X-Forwarded-For: 192.1.2.3
#
#       If you disable this, it will appear as
#
#               X-Forwarded-For: unknown
#
#Default:
# forwarded_for on
forwarded_for off

Reload squid.conf to make the change take effect

service squid reload