Remove placeholder file from releasenotes/notes dir

We have release notes now so we don't need a placeholder file to keep
the dir in the git tree anymore. This commit does just that and
removes the placeholder file.

Change-Id: I58be304d6341cf8795f268274e5e03aaeff6a695
diff --git a/releasenotes/notes/.placeholder b/releasenotes/notes/.placeholder
deleted file mode 100644
index e69de29..0000000
--- a/releasenotes/notes/.placeholder
+++ /dev/null