If you are using amsmath, just redefine \tagform@ in this way
\makeatletter
\renewcommand\tagform@[1]{\maketag@@@{\ignorespaces#1\unskip\@@italiccorr}}
\makeatother
MWE
\documentclass{article}
\usepackage{amsmath}
\makeatletter
\renewcommand\tagform@[1]{\maketag@@@{\ignorespaces#1\unskip\@@italiccorr}}
\makeatother
\begin{document}
\begin{equation}
x=y
\end{equation}
\end{document}

EDIT
As egreg notices, you might want \eqref to keep the parenthesis when printing the reference. In this case, replace the above code with
\makeatletter
\let\oldtagform@\tagform@
\renewcommand\tagform@[1]{\maketag@@@{\ignorespaces#1\unskip\@@italiccorr}}
\renewcommand{\eqref}[1]{\textup{\oldtagform@{\ref{#1}}}}
\makeatother
MWE:
\documentclass{article}
\usepackage{amsmath}
\makeatletter
\let\oldtagform@\tagform@
\renewcommand\tagform@[1]{\maketag@@@{\ignorespaces#1\unskip\@@italiccorr}}
\renewcommand{\eqref}[1]{\textup{\oldtagform@{\ref{#1}}}}
\makeatother
\begin{document}
\begin{equation}\label{myeq}
x=y
\end{equation}
A reference to equation \eqref{myeq}
\end{document}
Output:
