debian/rules: Don't overwrite .tarball-version
debian/rules: Don't overwrite .tarball-version

The .tarball-version file should contain the *source version* uniquely
identifying the git commit, and not the Debian package name.

With https://gerrit.osmocom.org/#/c/osmo-ci/+/10343/ there is a correct
.tarball-version file in the .tar.xz of the nightly source packages.

Change-Id: Idde432ffd3dae3b7beac77c794e60b6ba555e488
Related: OS#3449

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts