trafgen: Remove unused local variable in apply_*() functions.

The variable `i' in apply_counter(), apply_randomizer() and
apply_csum16() is only set once, so use the original parameter
`counter_id' instead. Rename `counter_id' to `id' in order to keep it
short.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>