Use Env#isPreInitializingContext() instead of a workaround

* It did not exist back then.

Author: eregon

src/main/java/org/truffleruby/RubyContext.java

@@ -151,18 +151,10 @@ public class RubyContext {
     private final AssumedValue<Boolean> warningCategoryDeprecated;
     private final AssumedValue<Boolean> warningCategoryExperimental;
 
-    private static boolean preInitializeContexts = TruffleRuby.PRE_INITIALIZE_CONTEXTS;
-
-    private static boolean isPreInitializingContext() {
-        boolean isPreInitializingContext = preInitializeContexts;
-        preInitializeContexts = false; // Only the first context is pre-initialized
-        return isPreInitializingContext;
-    }
-
     public RubyContext(RubyLanguage language, TruffleLanguage.Env env) {
         Metrics.printTime("before-context-constructor");
 
-        this.preInitializing = isPreInitializingContext();
+        this.preInitializing = env.isPreInitialization();
         this.preInitialized = preInitializing;
 
         preInitializationManager = preInitializing ? new PreInitializationManager() : null;

src/shared/java/org/truffleruby/shared/TruffleRuby.java

@@ -23,8 +23,6 @@ public class TruffleRuby {
     public static final String BOOT_SOURCE_NAME = "main_boot_source";
     public static final String RUBY_COPYRIGHT = "truffleruby - Copyright (c) 2013-" +
             BuildInformationImpl.INSTANCE.getCopyrightYear() + " Oracle and/or its affiliates";
-    public static final boolean PRE_INITIALIZE_CONTEXTS = System
-            .getProperty("polyglot.image-build-time.PreinitializeContexts") != null;
 
     public static String getVersionString(String implementationName) {
         final String buildName = BuildInformationImpl.INSTANCE.getBuildName();