Simplify HRTFDatabaseLoader's load map

Simplify HRTFDatabaseLoader's load map
https://bugs.webkit.org/show_bug.cgi?id=122944

Reviewed by Eric Carlson.

* platform/audio/HRTFDatabaseLoader.cpp:
(WebCore::loaderMap): Return a reference to a NeverDestroyed HashMap that maps sample rates to loaders.
(WebCore::HRTFDatabaseLoader::createAndLoadAsynchronouslyIfNecessary):
(WebCore::HRTFDatabaseLoader::~HRTFDatabaseLoader):
* platform/audio/HRTFDatabaseLoader.h: Remove the LoaderMap type definition, the private singleton of that type
and the singleton's unused getter.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@157806 268f45cc-cd09-0410-ab3c-d52691b4dbfc