You can't use a function in a converter. Generally, you would first parse the name and end up with 01, 02, etc. in the TRACK field. Then you would apply the function to clean up the TRACK field using an action. After using the Filename-Tag converter you could use a Format value action in either a saved Action group or through an Action (Quick):
Action type: Format value
Field: TRACK
Format string: $fmtNum(%track%)
OR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
If you had file names that looked something like:
01 - Ornithology
02 - Scrapple from the Apple
03 - Parker's Mood
then you could use the following format string in the Filename-Tag converter to remove the leading zeros, but it would only work on tracks 1 through 9
0%track% - %title%
You would need to use the following format string for tracks 10+:
%track% - %title%
This would work, but IMO it's not worth the trouble of having to select different ranges of tracks, changing the format string and doing two conversions.