BUG: Remove unnecessary fixture from #522

This block of code was making sure that we were no longer submitting invalid
investigation_types to EBI, but since #522 was merged, this is no longer
needed.

The problem was that the investigation type was being overwritten to
'Amplicon Sequencing' regardless of whatever the value was, so EBI was 
rejecting the submitted XML files with the following message:

```
<RECEIPT receiptDate="2014-11-14T16:53:02.221Z"
submissionFile="submission.xml" success="false">
   <SUBMISSION alias="qiime_submission_3"/>
       <MESSAGES>
           <ERROR>
               Study XML parse error: string value 'Amplicon Sequencing' is
not a valid enumeration value for type of existing_study_type attribute in
type of STUDY_TYPE element in type of DESCRIPTOR element in StudyType
           </ERROR>
           <ERROR>
               An exception occurred : string value 'Amplicon Sequencing' is
not a valid enumeration value for type of existing_study_type attribute in
type of STUDY_TYPE element in type of DESCRIPTOR element in StudyType
           </ERROR>
           <INFO>
               ADD action for the following XML: study.xml sample.xml
experiment.xml run.xml
           </INFO>
           </MESSAGES>
           <ACTIONS>ADD</ACTIONS>
           <ACTIONS>ADD</ACTIONS>
           <ACTIONS>ADD</ACTIONS>
           <ACTIONS>ADD</ACTIONS>
           <ACTIONS>HOLD</ACTIONS>
</RECEIPT>
```