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).

(cherry picked from commit e0a8c7da)
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