scripts: feeds: don't refresh .config upon update

scripts: feeds: don't refresh .config upon update

Feeds update does not make updated packages available to compile; they must be installed.
Despite this, update refreshes the .config, deleting selections in the .config which have not been
installed yet. The deleted selections are not restored with `./scripts/feeds install` nor with
`make defconfig` because these steps cannot conjure up already deleted selections.

Change update to not modify the .config and leave it to `./scripts/feeds install -d <y|m|n>` or
`make defconfig` or other *config options.

Signed-off-by: Ryan Leung <untilscour@protonmail.com>
Link: https://github.com/openwrt/openwrt/pull/23241
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>