Remove explicit `dirmngr` reference

This is pulled in automatically via `gnupg`, and moved from `Recommends` to `Depends` in 99474ad900, which has been part of `src:gnupg2` since 2.1.21-4 (and every supported version of both Debian _and_ Ubuntu have 2.2.x 😇).

Dockerfile.release

@@ -5,7 +5,7 @@ SHELL ["bash", "-Eeuo", "pipefail", "-xc"]
 RUN apt-get update; \
 	apt-get install -y --no-install-recommends \
 		file \
-		gnupg dirmngr \
+		gnupg \
 		wget \
 	; \
 	rm -rf /var/lib/apt/lists/*