GPX files are plain text and can be edited fine in Notepad etc (vi in my case).
The only thing to watch for is that you don't damage the pairings of start/end tags, so delete from "<trkpt>" up to and including "</trkpt>". Check that the file opens OK in Firefox or IE, and don't delete the original copy of the file until you have checked.
You can also merge tracks by editing in a similar way.