Proto: Fixed proto_unregister_field()

This function did not work after svn revision 51396.
This fix affects http, imf and ldap preferences.

Change-Id: I6fcb24c968d03c7d1fdc018cfa907f9cfd8809bb
Reviewed-on: https://code.wireshark.org/review/4881
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>