Upstream: no need to clear r->headers_out.last_modified_time.

Upstream: no need to clear r->headers_out.last_modified_time.

Clearing of the r->headers_out.last_modified_time field if a response
isn't cacheable in ngx_http_upstream_send_response() was introduced
in 3b6afa999c2f, the commit to enable not modified filter for cacheable
responses.  It doesn't make sense though, as at this point header was
already sent, and not modified filter was already executed.  Therefore,
the line was removed to simplify code.