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>
```
TST: Add more tests for EBI
TST: Use ENA_test instead of ENA
ENH: Appropriately use pop
Merge pull request #611 from ElDeveloper/ebi-bug-fix

BUG: Remove unnecessary fixture from #522