Drop committish prefix

As of I15ca67fb591, Gerrit adds the commit SHA1 to the messages in
ReceiveCommits.

Currently, error messages mention the commit SHA1 twice.

Change-Id: Ia83f27102448a10b5cb7f3d38502503e50178bf2

src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java b/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
index 7b0ee4e..35c87d5 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
@@ -64,11 +64,9 @@
   }
 
   private void onLineTooLong(
-      final AbbreviatedObjectId id,
       List<CommitValidationMessage> messagesList,
-      final String errorMessage)
+      final String message)
       throws CommitValidationException {
-    final String message = id.name() + ": " + errorMessage;
     if (rejectTooLong) {
       messagesList.add(new CommitValidationMessage(message, true));
       throw new CommitValidationException("Commit length validation failed", messagesList);
@@ -80,12 +78,10 @@
   public List<CommitValidationMessage> onCommitReceived(CommitReceivedEvent receiveEvent)
       throws CommitValidationException {
     final RevCommit commit = receiveEvent.commit;
-    final AbbreviatedObjectId id = commit.abbreviate(7);
     List<CommitValidationMessage> messages = new ArrayList<>();
 
     if (this.maxSubjectLength < commit.getShortMessage().length()) {
       onLineTooLong(
-          id,
           messages,
           "subject >" + this.maxSubjectLength + " characters; use shorter first paragraph");
     }
@@ -103,7 +99,6 @@
 
     if (longLineCnt > (longLinesThreshold * nonEmptyCnt) / 100) {
       onLineTooLong(
-          id,
           messages,
           "too many message lines longer than "
               + this.maxLineLength