Make blockdir always net specific

The blocks directory is net specific by definition.

Also this prevents the side effect of calling GetBlocksDir(false) in the
non-mainnet environment.

src/util/system.h

@@ -78,7 +78,8 @@ void ReleaseDirectoryLocks();
 
 bool TryCreateDirectories(const fs::path& p);
 fs::path GetDefaultDataDir();
-const fs::path &GetBlocksDir(bool fNetSpecific = true);
+// The blocks directory is always net specific.
+const fs::path &GetBlocksDir();
 const fs::path &GetDataDir(bool fNetSpecific = true);
 void ClearDatadirCache();
 fs::path GetConfigFile(const std::string& confPath);