Scripts/ToC: Simplify the code for "A Tribute to Immortality" achievement

* also fixes the achievement being still awarded if players die indirectly during boss fights (Anub'arak adds, for example)

(cherry picked from commit 9838cbda1a44ca6e45d114a0c2facc71cb481d38)
Merge branch '4.3.4' of https://github.com/TrinityCore/TrinityCore into 4.3.4