Makefile: Drop dead CONTAINER_RUNTIME

This variable is dead code as far as I can tell. I think it
got cargo culted from something similar in skopeo, where it *is*
used:
85598438ce/Makefile (L75)
etc.

(Instead it looks like there's a `PODMANCMD` here)

But I'm effectively using this PR as a way to suggest aligning
with what we're doing in bootc, where we have a core principle
that `Makefile` should *never* itself spawn containers (or VMs etc).
All the rules in Makefile are things that should work when e.g.
building RPMs or debs or the like.

Those tasks are things that are done via `Justfile`:
https://github.com/bootc-dev/bootc/blob/main/Justfile

So for example to align, we'd add a `Justfile` here and then
`make validatepr` would move to `just validatepr`.

Signed-off-by: Colin Walters <walters@verbum.org>

Makefile

@@ -96,7 +96,6 @@ endif
 SOURCES = $(shell find . -path './.*' -prune -o \( \( -name '*.go' -o -name '*.c' \) -a ! -name '*_test.go' \) -print) Makefile
 
 BUILDTAGS_CROSS ?= containers_image_openpgp exclude_graphdriver_btrfs exclude_graphdriver_overlay
-CONTAINER_RUNTIME := $(shell command -v podman 2> /dev/null || echo docker)
 OCI_RUNTIME ?= ""
 
 # The 'sort' below is crucial: without it, 'make docs' behaves differently