neorados: remove unused symlink to completion.h

this header is unused after ea67f3dee2a3f8fcdcbb0bc0e80e38ec70378f05
replaced uses of ceph::async::Completion with
boost::asio::any_completion_handler

Signed-off-by: Casey Bodley <cbodley@redhat.com>

diff --git a/src/include/neorados/completion.h b/src/include/neorados/completion.h
deleted file mode 120000 (symlink)
index 100678f..0000000
--- a/src/include/neorados/completion.h
+++ /dev/null
@@ -1 +0,0 @@
-../../common/async/completion.h
\ No newline at end of file