Revert "[Browser test] Reintroduce headless browser test support"

This reverts commit 8e8525dea9c0a37f9cff19d3ce5c7ed80595dd63.

This change seems to have broken all of the VisualEditor browser tests, similar to our first attempt at solving the same problem - not good :(

Change-Id: I5fa70a1164ebe8de1ecc37f72ba7ceffdc31e7bd
Merge "Revert "[Browser test] Reintroduce headless browser test support""