Remove noisy log statement for deleted release notes

I believe this `print()` statement was not intentional, as it's the only `print` to `sys.stderr` in the entire project. We already are logging to debug level, so we don't need to also print.

This issue causes ~20 log lines for Qiskit Terra any time we run Reno.

Change-Id: I664156017ea24e819c6a7cffe6711376306b59fe

releasenotes/notes/fix-noisy-log-deleted-release-notes-28208816eac8abe9.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Remove noisy log statement for deleted release notes. These look like:
+
+      unable to find release notes file associated with unique id '34cd623294a30d83', skipping

reno/scanner.py

@@ -15,7 +15,6 @@ import fnmatch
 import logging
 import os.path
 import re
-import sys
 
 from dulwich import diff_tree
 from dulwich import index as d_index
@@ -1225,7 +1224,6 @@ class Scanner(object):
                 msg = ('unable to find release notes file associated '
                        'with unique id %r, skipping') % uniqueid
                 LOG.debug(msg)
-                print(msg, file=sys.stderr)
 
         # Combine pre-releases into the final release, if we are told to
         # and the final release exists.