Tor Browser import doc: add missing cleanup step

wiki/src/contribute/release_process/tor-browser.mdwn

@@ -245,4 +245,5 @@ Clean up
 
 	cd "${TBB_ARCHIVE:?}" && \
 	git annex drop -- "${TBB_VERSION:?}" && \
-    rm -rf "${DL_DIR:?}"
+    rm -rf "${DL_DIR:?}" && \
+    rm "${TAILS_GIT_REPO:?}"/sha256sums-unsigned-build.txt{,.asc}