v3dv: drop a couple of obsolete comments

We only expose a coherent memory heap, so invalidation and flushing
are always no-ops for us.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>
diff --git a/src/broadcom/vulkan/v3dv_device.c b/src/broadcom/vulkan/v3dv_device.c
index cb246d5..a6751fc 100644
--- a/src/broadcom/vulkan/v3dv_device.c
+++ b/src/broadcom/vulkan/v3dv_device.c
@@ -1762,9 +1762,6 @@
                              uint32_t memoryRangeCount,
                              const VkMappedMemoryRange *pMemoryRanges)
 {
-   /* FIXME: stub (although note that both radv and tu just returns success
-    * here. Pending further research)
-    */
    return VK_SUCCESS;
 }
 
@@ -1773,9 +1770,6 @@
                                   uint32_t memoryRangeCount,
                                   const VkMappedMemoryRange *pMemoryRanges)
 {
-   /* FIXME: stub (although note that both radv and tu just returns success
-    * here. Pending further research)
-    */
    return VK_SUCCESS;
 }