I use the following sed command to remove the line number string from file
sed -i s\'/line number//g\' file
but what I need to
I'm not sure I fully understand your question, however this can help you :
sed -i '$ s/line number//g' file
This will perform substitution only on last line (because of $), replacing line number by nothing.
man sed, section Addresses can help you understand how to apply sed commands on part of the text.
For those who are on SunOS which is non-GNU, the following code will help:
sed '$d' tmp.dat > test.dat