Revert "Rename libsyn to libsyn2"

This reverts commit 7705ea6d36.

Reason for revert: I believe this is stale since `libsyn` is version 2 now:

https://cs.android.com/android/platform/superproject/+/main:external/rust/crates/syn/Android.bp;l=41;drc=fcd71dcc29ef325341ea7c1a145591b349018eb3

Change-Id: I31892447cf95d094e4bb201c4d86a28c70085467

scripts/cargo2android.py

@@ -83,11 +83,6 @@ RENAME_MAP = {
     'libx86_64': 'libx86_64_rust',
     'libxml': 'libxml_rust',
     'protoc_gen_rust': 'protoc-gen-rust',
-    # Due to incorrect branching by a downstream project, we cannot
-    # rename libsyn to libsyn_legacy or fully upgrade it to libsyn2
-    # in place. We change the default name to allow the legacy package
-    # to squat on the `libsyn` name.
-    'libsyn': 'libsyn2',
 }
 
 RENAME_STEM_MAP = {