remove comments on copyright status from UTF-8 implementation files

despite clarifications made to the COPYRIGHT file in commit
f0a6139933, there continues to be
confusion about whether the permissions granted actually apply to all
files. I am the sole author of these files and clearly intend, and
have always intended, for the grant of permission to apply to them.

src/multibyte/mbrlen.c

@@ -1,9 +1,3 @@
-/* 
- * This code was written by Rich Felker in 2010; no copyright is claimed.
- * This code is in the public domain. Attribution is appreciated but
- * unnecessary.
- */
-
 #include <wchar.h>
 
 size_t mbrlen(const char *restrict s, size_t n, mbstate_t *restrict st)