Don't put leapseconds.awk in tzdata tarball.

This harmless bug was introduced in release 2014f, which distributed
leapseconds.awk in both the code and the data tarballs.
It's code, so it should be in the code tarball.
* Makefile (DATA): Omit leapseconds.awk.
(check_character_set): Omit redundant check of $(LEAP_DEPS).