[core] Remove mutex from Style

There's no cross-thread sharing to be protected here.

src/mbgl/style/style.cpp

@@ -168,8 +168,6 @@ void Style::cascade() {
 }
 
 void Style::recalculate(float z) {
-    std::lock_guard<std::mutex> lock(mtx);
-
     for (const auto& source : sources) {
         source->enabled = false;
     }

src/mbgl/style/style.hpp

@@ -13,7 +13,6 @@
 #include <mbgl/util/worker.hpp>
 
 #include <cstdint>
-#include <mutex>
 #include <string>
 #include <vector>
 
@@ -137,7 +136,6 @@ private:
 
     std::exception_ptr lastError;
 
-    std::mutex mtx;
     ZoomHistory zoomHistory;
     bool hasPendingTransitions = false;