remove crate defined timezone constants

[oll3]

This PR removes the crate defined copy of chrono-tz timezone constants. This mainly to enable build-time timezone filtering (by the chrono-tz feature filter-by-regex). And this (again) mainly to decrease binary size (in similarity to #113).

I guess one draw back might be that you will need chrono_tz as a dependency to modify an rrule timezone. On the other hand the rrule lib could possibly re-export chrono_tz timezones to make it more convenient to work with.

I understand that this might not be wanted but since I will have this in my fork anyway I might as well create a PR and ask.