Fix bug 699450

* Remove extra call to gtk_tree_view_append_column. (It is already
  called in gnc_tree_view_account_add_custom_column.)

Author: Robert Ratliff <ratliff.bobby@gmail.com>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23175 57a11ea4-9604-0410-9ed3-97b8803252fd