[core] Fix readability-redundant-smartptr-get errors in header files

As reported by clang-tidy-8.

include/mbgl/style/color_ramp_property_value.hpp

@@ -27,7 +27,7 @@ public:
     ColorRampPropertyValue() : value(nullptr) {}
     ColorRampPropertyValue(std::shared_ptr<expression::Expression> value_) : value(std::move(value_)) {}
 
-    bool isUndefined() const { return value.get() == nullptr; }
+    bool isUndefined() const { return value == nullptr; }
 
     // noop, needed for batch evaluation of paint property values to compile
     template <typename Evaluator>

include/mbgl/util/image.hpp

@@ -56,9 +56,7 @@ public:
         return !(lhs == rhs);
     }
 
-    bool valid() const {
-        return !size.isEmpty() && data.get() != nullptr;
-    }
+    bool valid() const { return !size.isEmpty() && data != nullptr; }
 
     template <typename T = Image>
     T clone() const {