Examples: Remove PRNG.js

[Mugen87]

This file is not referenced in the entire repo.

[WestLangley]

@mrdoob added that a long time ago -- probably for its deterministic sequence.

If we want to compare screenshots someday, I expect a deterministic pseudo generator will be necessary.

There may be users who currently use this method.

[Mugen87]

Okay, if @mrdoob wants to keep PRNG, I suggest to move it into the THREE namespace and the file to the math directory.

[mrdoob]

We can always add it back if we need it.

[mrdoob]

Thanks!

[EliasHasle]

I used it. ;-)

But I am fine with it being removed, as the need is easily covered by alternatives found elsewhere, or by a custom implementation. It is not as if PRNG.js was some ingenious new PRNG algorithm. It was very simple and standard.