Remove the `output_to_genfiles = True` parameter from Starlark rules (#2512)

corresponding [g3 cl](https://critique.corp.google.com/cl/616227359)

BUG=N/A

third_party/flatbuffers/build_defs.bzl

diff --git a/third_party/flatbuffers/build_defs.bzl b/third_party/flatbuffers/build_defs.bzl
index f746b67..b26381e 100644
--- a/third_party/flatbuffers/build_defs.bzl
+++ b/third_party/flatbuffers/build_defs.bzl
@@ -365,7 +365,6 @@
             cfg = "exec",
         ),
     },
-    output_to_genfiles = True,
 )
 
 def flatbuffer_py_strip_prefix_srcs(name, srcs = [], strip_prefix = ""):
@@ -408,7 +407,6 @@
     attrs = {
         "deps": attr.label_list(mandatory = True),
     },
-    output_to_genfiles = True,
     outputs = {"out": "%{name}.py"},
 )