Simply use this perl command:
% perl -i.bak -npe 's/\r\n/\n/g' file ...
file is the file(s) to process. The modification is done in-place, with the original file stored with a .bak extension.
Alternatively you can use the tr(1) command:
% tr -d '\r' < dos-text-file > unix-file
dos-text-file is the file containing DOS text while unix-file will contain the converted output. This can be quite a bit faster than using perl.