docs: update README-hacking

We don't have a "README-valgrind" file.
So remove related description.
Signed-off-by: NChen Hanxiao <chenhanxiao@cn.fujitsu.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

README-hacking

@@ -15,7 +15,7 @@ Specific development tools and versions will be checked for and listed by
 the bootstrap script.
 
 Valgrind <http://valgrind.org/> is also highly recommended, if
-Valgrind supports your architecture. See also README-valgrind.
+Valgrind supports your architecture.
 
 While building from a just-cloned source tree may require installing a
 few prerequisites, later, a plain `git pull && make' should be sufficient.