Remove mutex lock on realizeTechniques

This was causing a deadlock - and I can't quite remember what problem it was intended to solve so it is best to remove it.

simgear/scene/material/Effect.cxx

@@ -1481,10 +1481,8 @@ void mergeSchemesFallbacks(Effect *effect, const SGReaderWriterOptions *options)
 
 // Walk the techniques property tree, building techniques and
 // passes.
-static std::mutex  realizeTechniques_lock;
 bool Effect::realizeTechniques(const SGReaderWriterOptions* options)
 {
-    std::lock_guard<std::mutex> g(realizeTechniques_lock);
     if (getPropertyRoot()->getBoolValue("/sim/version/compositor-support", false))
         mergeSchemesFallbacks(this, options);