(deprecate then?) remove the gzopen/gztell/gzseek shim

[mcdruid]

Per #47 (comment) a shim was added for an old PHP 5 bug.

It causes a warning when validating package.xml and is likely no longer required in the vast majority of cases.

Anyone using an affected PHP version could provide the shim another way.

[ashnazg]

Looking over this shim history, I'd say keep it until such time as you decide to drop PHP 5.5 at least. The validate warning will just be a reminder ;-)