[MC-181868] Gamerule randomTickSpeed doesn't affect fire spread anymore

For some reason, when I set the random tick speed, fire behaves like nothing changed.

Summon a large number of leaves and ensure that the "randomTickSpeed" gamerule is set to its default value, (3).
```
/fill ~3 ~ ~3 ~10 ~4 ~10 minecraft:oak_leaves[persistent=true]
/gamerule randomTickSpeed 3
```
Ignite the leaves, wait around thirty seconds, and take note of how fast the fire spread.
Once again, summon a large number of leaves, but this time, change the "randomTickSpeed" gamerule to a high value in an attempt to increase the speed of the fire spreading.
```
/fill ~-3 ~ ~-3 ~-10 ~4 ~-10 minecraft:oak_leaves[persistent=true]
/gamerule randomTickSpeed 300
```
Wait around thirty seconds and take note of how fast the fire spread.
Take note as to whether or not the "randomTickSpeed" gamerule affects the speed at which fire spreads.

The "randomTickSpeed" gamerule no longer affects the speed at which fire spreads.

The "randomTickSpeed" gamerule would affect the speed at which fire spreads, just like how it did in past versions of the game (1.12.2).

Code analysis by [Mod] Avoma can be found in this comment.