build: remove hand-written design BUILD files and stale references

Remove design BUILD.bazel files that are being replaced by
auto-generated targets from the orfs_design() macro in bazel-orfs.

Also remove stale merge_lib.py/preprocessLib.py references from
flow/util/BUILD.bazel (files were deleted in 644ff70 but the
BUILD reference was not updated).

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

Author: oharboe

flow/designs/asap7/aes/BUILD.bazel

@@ -34,9 +34,6 @@ filegroup(
 # for scripts/synth.sh steps
 filegroup(
     name = "makefile_yosys",
-    srcs = glob(MAKEFILE_SHARED) + [
-        "merge_lib.py",
-        "preprocessLib.py",
-    ],
+    srcs = glob(MAKEFILE_SHARED),
     visibility = ["//visibility:public"],
 )