docbook-sgml-dtd-native: remove catalog file when do_clean

docbook-sgml-dtd-native: remove catalog file when do_clean

The docbook_sgml_dtd_sstate_postinst adds catalog file to
/etc/sgml/sgml-docbook.cat and sgml-docbook.bak, but the do_clean would not
remove the file, which would cause unexpected errors since we have multiple
versions of docbook-sgml-dtd:

docbook-sgml-dtd-3.1-native_3.1.bb
docbook-sgml-dtd-4.1-native_4.1.bb
docbook-sgml-dtd-4.5-native.bb

If we run:

$ bitbake docbook-sgml-dtd-4.1-native
$ bitbake docbook-sgml-dtd-4.1-native -ccleansstate && bitbake docbook-sgml-dtd-3.1-native
$ bitbake docbook-utils-native

We would get this error:

jade: cannot open "/path/to/etc/sgml/sgml-docbook-dtd-4.1.cat" (No such file or directory)
make[2]: *** [sgmldiff.html] Error 1

Remove the catalog entry when do_clean will fix the problem.

[YOCTO #5949]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>