fix: remove stale plot_congestion target

The plot_congestion Bazel target and .bzl file are unused and cause
a stale reference to @orfs-pip requirements.

Cherry-pick of PR #4096.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

flow/util/BUILD.bazel

@@ -1,16 +1,7 @@
-load("@orfs-pip//:requirements.bzl", "requirement")
 load("@rules_python//python:pip.bzl", "compile_pip_requirements")
 
 exports_files(["open_plots.sh"])
 
-py_binary(
-    name = "plot_congestion",
-    srcs = ["plot_congestion.py"],
-    main = "plot_congestion.py",
-    visibility = ["//visibility:public"],
-    deps = [requirement("matplotlib")],
-)
-
 compile_pip_requirements(
     name = "requirements",
     src = "requirements.in",