maven_jar(): Remove gerrit specific condition

Snapshot artifacts on Maven Central have the same characteristics as
on Google storage buckets, so that these check shouldn't be Gerrit
repository specific [1]. In fact Bucklets are not Gerrit Code Review
project specific, but a generic plugin layer for Buck build system.

[1] https://github.com/davido/bucklets/issues/3
Change-Id: I05f138cb287e3021aa4282d7bd6e9dc4e7daae56