Bug 1372670 - part 2 - remove nsIThreadManager::isMainThread; r=erahm

Nobody calls this from JS, and we have better ways to accomplish the
same task in C++

xpcom/threads/nsIThreadManager.idl

@@ -75,12 +75,6 @@ interface nsIThreadManager : nsISupports
    */
   readonly attribute nsIThread currentThread;
 
-  /**
-   * This attribute is true if the calling thread is the main thread of the
-   * application process.
-   */
-  readonly attribute boolean isMainThread;
-
   /**
    * This queues a runnable to the main thread. It's a shortcut for JS callers
    * to be used instead of

xpcom/threads/nsThreadManager.cpp

@@ -338,15 +338,6 @@ nsThreadManager::GetCurrentThread(nsIThread** aResult)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-nsThreadManager::GetIsMainThread(bool* aResult)
-{
-  // This method may be called post-Shutdown
-
-  *aResult = (PR_GetCurrentThread() == mMainPRThread);
-  return NS_OK;
-}
-
 uint32_t
 nsThreadManager::GetHighestNumberOfThreads()
 {