Remove error message for missing post-sync hooks and accept string for sync-duration

- Remove the print statement that warns about a missing hook file
  in `post-sync.py`. This silences the error when a registered
  hook is not found.
- Change `--sync-duration-seconds` type from int to string (default) to accept
  values passed by repo tool.

Bug: 513002761
Test: ./run_tests
Flag: EXEMPT (PURE_REFACTOR)
Change-Id: I6c0833eca3f3905723b2a0952050f15278c65ff2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repohooks/+/583302
Commit-Queue: Ram Peri <ramperi@google.com>
Tested-by: Sam Saccone <samccone@google.com>
Reviewed-by: Sam Saccone <samccone@google.com>

post-sync.py

diff --git a/post-sync.py b/post-sync.py
index b796d59..de83774 100755
--- a/post-sync.py
+++ b/post-sync.py
@@ -127,11 +127,6 @@
             hook_path = repo_root_path / hook_path
 
         if not hook_path.exists():
-            print(
-                f"error: Registered post-sync hook '{name}' not found: "
-                f"{hook_path}",
-                file=sys.stderr,
-            )
             return 1
 
         # Replace the first element with the resolved path.
@@ -193,7 +188,7 @@
     )
     parser.add_argument(
         "--sync-duration-seconds",
-        type=int,
+        default="",
         help="The total time taken by the sync operation.",
     )