Remove saving a function pointer only used once

This removes the global struct member for
vkEnumerateInstanceExtensionProperties, which is only used in the same
function that fetches the function from the loader.

The pattern is now more in line with how other single uses of Vulkan
functions are structures, such as window surface creation.

src/internal.h

@@ -329,7 +329,6 @@ typedef void (APIENTRY * PFN_vkVoidFunction)(void);
 #else
   typedef PFN_vkVoidFunction (APIENTRY * PFN_vkGetInstanceProcAddr)(VkInstance,const char*);
   typedef VkResult (APIENTRY * PFN_vkEnumerateInstanceExtensionProperties)(const char*,uint32_t*,VkExtensionProperties*);
-  #define vkEnumerateInstanceExtensionProperties _glfw.vk.EnumerateInstanceExtensionProperties
   #define vkGetInstanceProcAddr _glfw.vk.GetInstanceProcAddr
 #endif
 
@@ -855,7 +854,6 @@ struct _GLFWlibrary
         void*           handle;
         char*           extensions[2];
 #if !defined(_GLFW_VULKAN_STATIC)
-        PFN_vkEnumerateInstanceExtensionProperties EnumerateInstanceExtensionProperties;
         PFN_vkGetInstanceProcAddr GetInstanceProcAddr;
 #endif
         GLFWbool        KHR_surface;

src/vulkan.c

@@ -45,6 +45,7 @@ GLFWbool _glfwInitVulkan(int mode)
 {
     VkResult err;
     VkExtensionProperties* ep;
+    PFN_vkEnumerateInstanceExtensionProperties vkEnumerateInstanceExtensionProperties;
     uint32_t i, count;
 
     if (_glfw.vk.available)
@@ -81,9 +82,9 @@ GLFWbool _glfwInitVulkan(int mode)
         return GLFW_FALSE;
     }
 
-    _glfw.vk.EnumerateInstanceExtensionProperties = (PFN_vkEnumerateInstanceExtensionProperties)
+    vkEnumerateInstanceExtensionProperties = (PFN_vkEnumerateInstanceExtensionProperties)
         vkGetInstanceProcAddr(NULL, "vkEnumerateInstanceExtensionProperties");
-    if (!_glfw.vk.EnumerateInstanceExtensionProperties)
+    if (!vkEnumerateInstanceExtensionProperties)
     {
         _glfwInputError(GLFW_API_UNAVAILABLE,
                         "Vulkan: Failed to retrieve vkEnumerateInstanceExtensionProperties");