Replace uses of boost::filesystem with fs

Step two in abstracting away boost::filesystem.

To repeat this, simply run:
```
git ls-files \*.cpp \*.h | xargs sed -i 's/boost::filesystem/fs/g'
```

src/qt/test/rpcnestedtests.cpp

@@ -155,5 +155,5 @@ void RPCNestedTests::rpcNestedTests()
     delete pblocktree;
     pblocktree = nullptr;
 
-    boost::filesystem::remove_all(boost::filesystem::path(path));
+    fs::remove_all(fs::path(path));
 }