Remove redundant includes. Conform to header include guidelines.

From the header include guidelines (developer-notes.md):

"One exception is that a `.cpp` file does not need to re-include the
 includes already included in its corresponding `.h` file."

* rpc/util.h includes pubkey.h + utilstrencodings.h. rpc/util.cpp includes rpc/util.h.
* util.h includes fs.h. util.cpp includes util.h.

src/util.cpp

@@ -4,7 +4,6 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <util.h>
-#include <fs.h>
 
 #include <chainparamsbase.h>
 #include <random.h>