layers: Remove dead render_pass forward declaration

This went away at the same time as the SC/DS merge.

Signed-off-by: Chris Forbes <chrisforbes@google.com>
diff --git a/layers/core_validation.cpp b/layers/core_validation.cpp
index d950dc0..1a0499f 100644
--- a/layers/core_validation.cpp
+++ b/layers/core_validation.cpp
@@ -99,7 +99,6 @@
 
 // fwd decls
 struct shader_module;
-struct render_pass;
 
 struct layer_data {
     debug_report_data *report_data;