Remove unnecessary imports of text-fixture.html

[limonte]

Fixes #193

There's no <link rel="import" href="../../test-fixture/test-fixture.html"> in skeleton, once this PR is approved and merged, I'm going to fix the same thing in all other components.

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This change is 

[web-padawan]

Reviewable status: 0 of 9 files reviewed, 1 unresolved discussion (waiting on @limonte and @web-padawan)

a discussion (no related file):
Let's revert vaadin/vaadin-element-skeleton#165 and stick to the following order:

  <script src="../../web-component-tester/browser.js"></script>
  <script src="../../webcomponentsjs/webcomponents-lite.js"></script>
  <link rel="import" href="../../test-fixture/test-fixture.html">

Note: the Symbol polyfill issue, which happens in IE11 when placing browser.js after the polyfill, is being tracked at webcomponents/webcomponentsjs#1047

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