Remove unused condition seeding and the supporting xml/yml methods

EDIT: Added commit references for when we disabled shipping default conditions

**condition.rb, now almost 40% smaller!** :scissors: :scissors: 

YAMLImportExportMixin is now responsible for importing conditions and only calls Condition#export_to_array or Condition#import_from_hash.

All of the other stuff is unused.  We supported importing Conditions from yaml and xml via seeding at startup by looking in product/conditions directory.  The yaml files in that directory were all renamed to .disabled a while ago so they weren't used.  I don't know that we ever used import/exporting conditions in xml.

I have an old Xav database that I was able to successfully to export/import Policies with Conditions using this pull request.

**Commits when we disabled the default condition yamls**:

```
commit 553dc9a0a452d223fd45395810daf4f58b51ad58
Date:   Mon May 12 22:24:47 2008 +0000

    BugzID: 2614
    -Disabled creation of default, out of the box, sample policies.
    -Disabled creation of all default sample policy conditions except: always, Sample: VM Linux OS, Sample: VM Windows OS.

commit 64c2e182c2aa1e61edcc7ea09eb2c7247ac46cbb
Date:   Fri Apr 3 14:11:35 2009 +0000

    BugzID: 5332
    Disabled "Always" sample condition since it is now implied when no conditions are connected to a policy.

commit ab54601f6585390e92131a3d24476b7ea903a3ff
Date:   Mon Apr 13 22:28:16 2009 +0000

    Allowed deletion of sample conditions and removed preload of win_only and linux_only sample conditions as per PM.
```