[antlir1] Remove eln flavor

Summary: All eln image builds have moved to antlir2 so remove antlir1 eln support.

Test Plan:
Verify target graph:
```
buck2 cquery '
    deps(//antlir/...) +
    deps(//bot_generated/antlir/...) +
    deps(//metalos/...)
    deps(//os_foundation/...) +
    deps(//tupperware/...) +
'
```

Past that:
waitforsandcastle

Reviewed By: vmagro

Differential Revision: D50060423

fbshipit-source-id: c96932f8f346edfdd5f604e2d2e7e1aeb55cfe50

antlir/BUCK

@@ -2,7 +2,9 @@
 
 load("//antlir/bzl:build_defs.bzl", "buck_genrule", "buck_sh_binary", "export_file", "python_binary", "python_library", "python_unittest", "rust_binary", "rust_library", "third_party")
 load("//antlir/bzl:constants.bzl", "BZL_CONST", "REPO_CFG")
+load("//antlir/bzl:flavor_helpers.bzl", "flavor_helpers")
 load("//antlir/bzl:gpt.shape.bzl", "gpt_partition_t", "gpt_t")
+load("//antlir/bzl:image.bzl", "image")
 load("//antlir/bzl:image_python_unittest.bzl", "image_python_unittest")
 load("//antlir/bzl:layer_resource.bzl", "layer_resource")
 load("//antlir/bzl:query.bzl", "query")
@@ -895,6 +897,12 @@ buck_sh_binary(
     visibility = ["PUBLIC"],
 )
 
+image.layer(
+    name = "empty.layer",
+    flavor = flavor_helpers.get_flavor_default(),
+    visibility = ["PUBLIC"],
+)
+
 python_binary(
     name = "vcs",
     srcs = ["vcs.py"],