[runtimes][NFC] Remove filenames at the top of the license notice

We've stopped doing it in libc++ for a while now because these names
would end up rotting as we move things around and copy/paste stuff.
This cleans up all the existing files so as to stop the spreading
as people copy-paste headers around.
diff --git a/libcxx/src/future.cpp b/libcxx/src/future.cpp
index 4c59f89..177fe75 100644
--- a/libcxx/src/future.cpp
+++ b/libcxx/src/future.cpp
@@ -1,4 +1,4 @@
-//===------------------------- future.cpp ---------------------------------===//
+//===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.