What you have to do is to

Open the original file for input
Open a new file for output.
Read a line from the original file.
Check to see if this is a line you want to delete.  If it is, then skip the
next step.
Write the line to the new output file.
Go back to see if there are more lines in the original file to be read.  If
not, do the following steps
Close both files, KILL the original file (or rename it - much safer)
Rename the new file to the original file's old name.

Done.

Tom Lake