sed 's/[ \t]*$//'
NOTE: The expression '\t' is used to indicate a tab character (0x09) in the scripts.
Some versions of sed do not recognize the '\t' abbreviation, so when typing these scripts from the command line, you should press the TAB key instead.
'\t' is supported as a regular expression.
cat input.txt | sed 's/[ \t]*$//' > output.txt