Removed bump to api 26 for validation layers.

Author: gaaclarke

tools/gn

@@ -648,8 +648,6 @@ def to_gn_args(args):
 
   if enable_vulkan_validation:
     gn_args['enable_vulkan_validation_layers'] = True
-    if args.target_os == 'android':
-      gn_args['android_api_level'] = 26
 
   # Enable pointer compression on 64-bit mobile targets. iOS is excluded due to
   # its inability to allocate address space without allocating memory.