Revert "add BoardConfig flag TARGET_USES_OPENGLES_FOR_SCREEN_CAPTURE"

This reverts commit db66fbf4acbe098dc52534378ed33694a63f3ffd.
Turns out the test results in the GN were skewed by another bug. We _can_ just remove the useReadPixels check and do away with the need for a board flag

Change-Id: I084e7bbf4c2435b9656fa30e7ce130237b7f57c5