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fix getLinkTarget for MEI 5 work titles (#505)

## Description, Context and related Issue

Remove `mei:titleStmt` when determining the title of the work because in
MEI 5 `mei:title` is a direct child of `mei:work`
Closes #337 

## How Has This Been Tested?

With a BAZ-GA MEI 5 work.

## Types of changes

- Bug fix (non-breaking change which fixes an issue)

## Overview
<!--- Go over all the following points, and DELETE options that are not
relevant. -->
<!--- If you're unsure about any of these, don't hesitate to ask. We're
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- I have performed a self-review of my code, according to the [style
guide](https://github.com/Edirom/Edirom-Online/blob/develop/STYLE-GUIDE.md)
- I have read the
[CONTRIBUTING](https://github.com/Edirom/Edirom-Online/blob/develop/CONTRIBUTING.md)
document.
- I have added tests to cover my changes at
[testing](https://github.com/Edirom/Edirom-Online/tree/develop/testing)

- All new and existing tests passed.
--> will know when the PR has run its tests ;-)