Remove -detachdb and stop's detach argument.

As the only BDB database left is the wallet, and it is always
detached.

Also remove IsChainFile() predicate and related chainfile-specific
logic.

src/db.h

@@ -31,7 +31,6 @@ bool BackupWallet(const CWallet& wallet, const std::string& strDest);
 class CDBEnv
 {
 private:
-    bool fDetachDB;
     bool fDbEnvInit;
     bool fMockDb;
     boost::filesystem::path pathEnv;
@@ -71,8 +70,6 @@ public:
     void Close();
     void Flush(bool fShutdown);
     void CheckpointLSN(std::string strFile);
-    void SetDetach(bool fDetachDB_) { fDetachDB = fDetachDB_; }
-    bool GetDetach() { return fDetachDB; }
 
     void CloseDb(const std::string& strFile);
     bool RemoveDb(const std::string& strFile);