Remove setting backPath and 7.6.x workaround

[thomaszbz]

MetaSEO version: 3.0.0
TYPO3 version: 8.7.x
PHP version: *

This is a follow-up to #150 which resulted in a workaround: Setting the backPath when instanciating TSFE.

Setting the backPath is not needed any more since 8.0 since setting the backPath does not have any effect any more. As long as this holds true, setting the backPath can be removed as well as the workaround described in #150 .

• https://forge.typo3.org/issues/69319
• https://docs.typo3.org/typo3cms/extensions/core/Changelog/8.0/Breaking-73763-RemovedBackPathFromPageRenderer.html
• 247ef20