All actionview caches are already cleared at the start of each request
(when Resolver.caching is false) by PerExecutionDigestCacheExpiry, which
calls LookupContext::DetailsKey.clear (which clears all caches).

Because caches are always cleared per-request in dev, we shouldn't need
this extra logic to compare mtimes and conditionally reload templates.

This should make templates slightly faster in development (particularly
multiple renders of the same template)