Revert "BUG #15874: Fix texture blending to discard transparent pixels."
This reverts commit 2c3f90a2. This fixes BUG #15993. Discarding fragments with 0 alpha ended up discarding pixels even when blending was disabled. That was causing the bug reported in 159993. BUG #15874 has since been fixed by paraview/paraview!598, this fix is no longer needed for the issue it was added to fix (albeit incorrectly).