Remove is_buck2() calls from antlir/bzl/flatten.bzl

Summary:
buck1 is declared dead, is_buck2() checks will always return true. We can statically eliminate
these calls and associated conditional checks. antlir/bzl/flatten.bzl was codemodded with libcst.

Test Plan: Sandcastle

Reviewed By: dtolnay

Differential Revision: D63295334

fbshipit-source-id: 6b811b52458af552130101afc3fb7045df3e9b94

antlir/bzl/flatten.bzl

@@ -6,7 +6,6 @@
 # This does incorrect modifications.
 # @lint-ignore-every FBCODEBZLADDLOADS
 
-load("//antlir/bzl:build_defs.bzl", "is_buck2")
 load("//antlir/bzl:types.bzl", "types")
 
 def _flatten_any(lst):
@@ -23,12 +22,11 @@ def _typed_flattener(item_type) -> types.function:
  types.lint_noop(item_type)
 
  # @lint-ignore BUCKLINT
- t = native.eval_type(list[item_type]) if is_buck2() else ""
+ t = native.eval_type(list[item_type])
 
  def _flatten(lst):
  r = _flatten_any(lst)
- if is_buck2():
- t.check_matches(r)
+ t.check_matches(r)
  return r
 
  return _flatten