Revert "Properly setup seed in RNG"

This reverts commit 1dd72dca45.
As pointed out in #27171, it would break compatibility with 3.1.0.

core/math/random_pcg.cpp

@@ -34,7 +34,8 @@
 
 RandomPCG::RandomPCG(uint64_t p_seed, uint64_t p_inc) :
 		pcg(),
-		current_inc(p_inc) {
+		current_seed(DEFAULT_SEED) {
+	pcg.inc = p_inc;
 	seed(p_seed);
 }