Remove dead UV_THREADPOOL_SIZE assignment from gulpfile (#53490)

gulpfile.js set process.env.UV_THREADPOOL_SIZE, but gulp's CLI loads the
gulpfile through liftoff, which does async fs work first. libuv has already
sized its threadpool to the default 4 by then, so the assignment never took
effect. Measured through the gulp CLI, effective threadpool size is 3.4 both
with and without the line, and only reaches 6.9 when the variable comes from
the environment.

Hoisting it earlier cannot fix it either, since the pool is sized before any
line of gulpfile.js runs, and setting it for real measurably changes nothing.
Remove it rather than plumb the variable through the build.

Author: MindFreeze

gulpfile.js

@@ -1,4 +1 @@
-import { availableParallelism } from "node:os";
 import "./build-scripts/gulp/index.mjs";
-
-process.env.UV_THREADPOOL_SIZE = availableParallelism();