archrelease: Fix removal of repository content

Fix the `svn rm` invocation when releasing to a repository that already
contains some files. `svn ls` only returns basenames, so we need to
prepend the path here.

Regression introduced in 8384ad84.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>