The beamer theme Dresden uses a so-called mini frame navigation, i. e. little circles each representing one slide. With \setbeamertemplate{mini frames}{}, you can remove these, but the space is still occupied:

To get rid of this gap, you have two possibilities:
Remove all \subsection commands from your presentation. Mini frames are only created if there is a subsection present.
Add \renewcommand*{\slideentry}[6]{} to the preamble of your document (the part between \documentclass{beamer} and \begin{document}). This empties the internal beamer macro responsible for generating the mini frames and the space around them:
\documentclass{beamer}
\usetheme{Dresden}
\setbeamertemplate{navigation symbols}{}
\setbeamertemplate{mini frames}{}
\renewcommand*{\slideentry}[6]{}
\begin{document}
\section{Section 1}
\subsection{Subsection 1}
\frame{}\frame{}\frame{}
\subsection{Subsection 2}
\frame{}\frame{}
\section{Section 2}
\subsection{Subsection 1}
\frame{}\frame{}
\subsection{Subsection 2}
\frame{}\frame{}\frame{}\frame{}\frame{}\frame{}
\end{document}
