man: drop mention of /usr/include/ from file-hierarchy(7) man page

The man page is supposed to provide a "generalized, though minimal and
modernized subset" (as per introductory pargapraghs), from a systemd
perspective. But the thing is that /usr/include/ really doesn't matter
to us. It's a development thing, and slightly weird (because it arguably
would be better places in /usr/share/include/ or so). It's not going to
be there on 95% of deployed systems, and we really don't want people to
bother with it on such systems.

We only define the skeleton of directories in this document, and it's
expected that people extend it, and I think this really should be one of
those dirs that is an extension of our skeleton, but not part of the
skeleton, if that makes any sense.

man/file-hierarchy.xml

@@ -182,11 +182,6 @@
         subdirectory of <filename>/usr/lib/</filename> instead.</para></listitem>
       </varlistentry>
 
-      <varlistentry>
-        <term><filename>/usr/include/</filename></term>
-        <listitem><para>C and C++ API header files of system libraries.</para></listitem>
-      </varlistentry>
-
       <varlistentry>
         <term><filename>/usr/lib/</filename></term>
         <listitem><para>Static, private vendor data that is compatible with all architectures (though not
@@ -616,10 +611,6 @@
       <entry><filename>/usr/lib/<replaceable>arch-id</replaceable>/<replaceable>package</replaceable>/</filename></entry>
       <entry>Private other vendor resources of the package that are architecture-specific and cannot be shared between architectures. Note that this generally does not include private executables since binaries of a specific architecture may be freely invoked from any other supported system architecture.</entry>
           </row>
-          <row>
-      <entry><filename>/usr/include/<replaceable>package</replaceable>/</filename></entry>
-      <entry>Public C/C++ APIs of public shared libraries of the package.</entry>
-          </row>
         </tbody>
       </tgroup>
     </table>