linker: delete plan9 support

This experimental target was never fully completed. The operating system
is not that interesting or popular anyway, and the maintainer is no
longer around.

Not worth the maintenance burden. This code can be resurrected later if
it is worth it. In such case it will be subject to greater scrutiny.

src/codegen.zig

@@ -997,8 +997,6 @@ pub fn genNavRef(
             },
             .link_once => unreachable,
         }
-    } else if (lf.cast(.plan9)) |p9| {
-        return .{ .sym_index = try p9.seeNav(pt, nav_index) };
     } else {
         const msg = try ErrorMsg.create(zcu.gpa, src_loc, "TODO genNavRef for target {}", .{target});
         return .{ .fail = msg };