net: rename Sock::Reset() to Sock::Close() and make it private

Outside of `Sock`, `Sock::Reset()` was used in just one place (in
`i2p.cpp`) which can use the assignment operator instead.

This simplifies the public `Sock` API by having one method less.

src/test/util/net.h

@@ -100,7 +100,7 @@ public:
         m_socket = INVALID_SOCKET - 1;
     }
 
-    ~StaticContentsSock() override { Reset(); }
+    ~StaticContentsSock() override { m_socket = INVALID_SOCKET; }
 
     StaticContentsSock& operator=(Sock&& other) override
     {
@@ -108,11 +108,6 @@ public:
         return *this;
     }
 
-    void Reset() override
-    {
-        m_socket = INVALID_SOCKET;
-    }
-
     ssize_t Send(const void*, size_t len, int) const override { return len; }
 
     ssize_t Recv(void* buf, size_t len, int flags) const override