[api] Don't provide a default PostJob implementation

The default implementation just returns {nullptr}, which is not a
correct implementation. Since we rely on the implementation of {PostJob}
since https://crrev.com/c/2301933, and embedders can just use
{NewDefaultJobHandle} since https://crrev.com/c/2304812 (backported to
8.5), we should stop providing this dangerous default.

R=ulan@chromium.org

Bug: v8:10723, v8:10740, chromium:1101340
Change-Id: I6e34c584cbed186ddf6cfa4a9c5a7e8caa3b61c8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2315981
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69021}

include/v8-platform.h

@@ -540,9 +540,7 @@ class Platform {
    * }
    */
   virtual std::unique_ptr<JobHandle> PostJob(
-      TaskPriority priority, std::unique_ptr<JobTask> job_task) {
-    return nullptr;
-  }
+      TaskPriority priority, std::unique_ptr<JobTask> job_task) = 0;
 
   /**
    * Monotonically increasing time in seconds from an arbitrary fixed point in