Cycles OpenCL: Remove single program

Part of the cleanup of the OpenCL codebase.
Single program is not effective when using OpenCL, it is slower
to compile and slower during rendering (when used in for example
`barbershop` or `victor`).

Reviewers: brecht, #cycles

Maniphest Tasks: T62267

Differential Revision: https://developer.blender.org/D4481

intern/cycles/util/util_debug.cpp

@@ -90,8 +90,7 @@ void DebugFlags::CUDA::reset()
 
 DebugFlags::OpenCL::OpenCL()
   : device_type(DebugFlags::OpenCL::DEVICE_ALL),
-    debug(false),
-    single_program(false)
+    debug(false)
 {
 	reset();
 }
@@ -123,7 +122,6 @@ void DebugFlags::OpenCL::reset()
 	}
 	/* Initialize other flags from environment variables. */
 	debug = (getenv("CYCLES_OPENCL_DEBUG") != NULL);
-	single_program = (getenv("CYCLES_OPENCL_SINGLE_PROGRAM") != NULL);
 }
 
 DebugFlags::DebugFlags()
@@ -179,7 +177,6 @@ std::ostream& operator <<(std::ostream &os,
 	os << "OpenCL flags:\n"
 	   << "  Device type    : " << opencl_device_type << "\n"
 	   << "  Debug          : " << string_from_bool(debug_flags.opencl.debug) << "\n"
-	   << "  Single program : " << string_from_bool(debug_flags.opencl.single_program) << "\n"
 	   << "  Memory limit   : " << string_human_readable_size(debug_flags.opencl.mem_limit) << "\n";
 	return os;
 }