alembic: Drop redundant voicemail_messages index.

The `voicemail_messages_dir` index is a left prefix of the table's
primary key and therefore unnecessary.

(cherry picked from commit d9c14cba49)

contrib/ast-db-manage/voicemail/versions/64fae6bbe7fb_drop_redundant_index.py

@@ -0,0 +1,24 @@
+"""Drop redundant index
+
+Revision ID: 64fae6bbe7fb
+Revises: 1c55c341360f
+Create Date: 2024-09-26 16:17:12.732445
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '64fae6bbe7fb'
+down_revision = '1c55c341360f'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    with op.batch_alter_table('voicemail_messages') as batch_op:
+        batch_op.drop_index('voicemail_messages_dir')
+
+
+def downgrade():
+    with op.batch_alter_table('voicemail_messages') as batch_op:
+        batch_op.create_index('voicemail_messages_dir', ['dir'])