Remove ReadVersion and WriteVersion

The "version" record that these functions read and write are not
used anywhere in the code except for one place. There is no reason
to expose these functions publicly. Furthermore, this avoids potential
confusion as developers may mistake these functions for actually
reading and writing the wallet version when they do not.

src/wallet/db.cpp

@@ -587,7 +587,7 @@ BerkeleyBatch::BerkeleyBatch(BerkeleyDatabase& database, const char* pszMode, bo
             if (fCreate && !Exists(std::string("version"))) {
                 bool fTmp = fReadOnly;
                 fReadOnly = false;
-                WriteVersion(CLIENT_VERSION);
+                Write(std::string("version"), CLIENT_VERSION);
                 fReadOnly = fTmp;
             }
         }