Revert "Use one context per OSL thread. Not sure if this actually works, but the simple renderer example in OSL does it this way."

This does not actually work: The context must not be shared between threads, but using the same context between different samples actually seems to prevent OSL from switching between shaders. The proper solution would be to ensure memory pooling works correctly.

This reverts commit 69f87e69258d6266dcb20f09f7e3d4021e663432.

intern/cycles/kernel/osl/osl_globals.h

@@ -64,7 +64,6 @@ struct OSLGlobals {
 	struct ThreadData {
 		OSL::ShaderGlobals globals;
 		OSL::PerThreadInfo *thread_info;
-		OSL::ShadingContext *ctx;
 	};
 
 	static tls_ptr(ThreadData, thread_data);