Remove stale forward declarations (#662)

diff --git a/src/format_parser.h b/src/format_parser.h
index 9457130..a57eb44 100644
--- a/src/format_parser.h
+++ b/src/format_parser.h
@@ -23,8 +23,6 @@
 
 namespace amber {
 
-class Format;
-
 /// Parses a Vulkan image string into a format object.
 class FormatParser {
  public:
diff --git a/src/vulkan/buffer_descriptor.h b/src/vulkan/buffer_descriptor.h
index bb4e408..3c6a690 100644
--- a/src/vulkan/buffer_descriptor.h
+++ b/src/vulkan/buffer_descriptor.h
@@ -26,9 +26,6 @@
 #include "src/vulkan/transfer_buffer.h"
 
 namespace amber {
-
-class Format;
-
 namespace vulkan {
 
 class CommandBuffer;
diff --git a/src/vulkan/resource.h b/src/vulkan/resource.h
index 81ba8df..cb94ecd 100644
--- a/src/vulkan/resource.h
+++ b/src/vulkan/resource.h
@@ -23,9 +23,6 @@
 #include "amber/vulkan_header.h"
 
 namespace amber {
-
-class Format;
-
 namespace vulkan {
 
 class CommandBuffer;