Remove override of unpack(self)

I cargo culted that from my *nextflow* package.  I [thought I] needed it
to work around Spack trying to use tar to unpack something that was
neither a tar ball nor unpackable.

This package works fine without it.  In retrospect, the error that I was
seeing in the *nextflow* package was probably this problem #1308.

var/spack/repos/builtin/packages/ack/package.py

@@ -43,9 +43,6 @@ class Ack(Package):
     # should:
     # depends_on('perl')
 
-    def unpack(self):
-        pass
-
     def install(self, spec, prefix):
         mkdirp(prefix.bin)
         ack = 'ack-{0}-single-file'.format(self.version)