d/gbp.conf: Drop v from pureos version

Make this consistent with other packages
5 jobs for gbp.conf in 9 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Package
passed #357458
librem5
package-pureos-byzantium

00:04:36

passed #357459
librem5:arm64
package-pureos-byzantium:arm64

00:09:01

 
  Test Package
passed #357460
librem5
autopkgtest-pureos-byzantium-package

00:00:58

passed #357462
librem5
check-tarball

00:02:20

passed #357461
librem5
lintian-pureos-byzantium-package

00:00:53