expand_template: do not output_to_genfiles (#493)

output_to_genfiles is deprecated unless needed for backwards compatibility
with legacy rules (see https://bazel.build/rules/lib/globals/bzl#rule)

rules/expand_template.bzl

diff --git a/rules/expand_template.bzl b/rules/expand_template.bzl
index f933500..f95507e 100644
--- a/rules/expand_template.bzl
+++ b/rules/expand_template.bzl
@@ -46,5 +46,4 @@
             doc = "The destination of the expanded file.",
         ),
     },
-    output_to_genfiles = True,
 )