maven_jar: Remove unused unsign attribute

Last usage of unsign artifacts machinery was removed in I3be17767ed4,
but the unsign attribute was missed to be removed in maven_jar rule.

Change-Id: Ib925ddf4c65aa73e9cbeec02cf4c57c296a7a722
diff --git a/tools/bzl/maven_jar.bzl b/tools/bzl/maven_jar.bzl
index c8ba7ca..177e80b 100644
--- a/tools/bzl/maven_jar.bzl
+++ b/tools/bzl/maven_jar.bzl
@@ -170,7 +170,6 @@
         "repository": attr.string(default = MAVEN_CENTRAL),
         "sha1": attr.string(),
         "src_sha1": attr.string(),
-        "unsign": attr.bool(default = False),
         "exports": attr.string_list(),
         "deps": attr.string_list(),
         "_download_script": attr.label(default = Label("//tools:download_file.py")),