Simplify initialization of static class properties in the worker-thread

Now that we no longer depend on the old Babel version in SystemJS we can remove the `static get ...` work-arounds used to define constants, which leads to slightly more compact code.

src/core/function.js

@@ -513,9 +513,7 @@ function isPDFFunction(v) {
 }
 
 class PostScriptStack {
-  static get MAX_STACK_SIZE() {
-    return shadow(this, "MAX_STACK_SIZE", 100);
-  }
+  static MAX_STACK_SIZE = 100;
 
   constructor(initialStack) {
     this.stack = initialStack ? Array.from(initialStack) : [];