vulkan: Remove bogus IMAGE_USAGE_GENERAL from todo note

There is no such usage. There is also no risk of bad interactions with
cpu-mapping, since gralloc-backed images do not use a VkDeviceMemory.

Test: build
Change-Id: I2d7512ec79777f796826bc2b9c8dc3626a535737
diff --git a/vulkan/libvulkan/swapchain.cpp b/vulkan/libvulkan/swapchain.cpp
index e069b5d..b49a06c 100644
--- a/vulkan/libvulkan/swapchain.cpp
+++ b/vulkan/libvulkan/swapchain.cpp
@@ -330,7 +330,6 @@
     // TODO(jessehall): I think these are right, but haven't thought hard about
     // it. Do we need to query the driver for support of any of these?
     // Currently not included:
-    // - VK_IMAGE_USAGE_GENERAL: maybe? does this imply cpu mappable?
     // - VK_IMAGE_USAGE_DEPTH_STENCIL_BIT: definitely not
     // - VK_IMAGE_USAGE_TRANSIENT_ATTACHMENT_BIT: definitely not
     capabilities->supportedUsageFlags =