interfaces: remove _defaultgw use altogether

[fichtner]

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Is your feature request related to a problem? Please describe.

system_default_route() is the authoritative source of _defaultgw files, but for historical reasons the following spots seem to be injecting the file leading to multiple files in multi-WAN setup.

src/etc/inc/interfaces.inc:    file_put_contents("/tmp/{$stfiface}_defaultgwv6", "{$rd6brgw}\n"); /* XXX omit in the future */
src/etc/inc/interfaces.inc:    file_put_contents("/tmp/{$stfiface}_defaultgwv6", "{$stfbrgw}\n"); /* XXX omit in the future */
src/etc/inc/interfaces.inc:    echo \${2} > /tmp/\${1}_defaultgwv6

Plus some other manual use of the file should be audited.

~~If there is still time also see if this can be removed:~~ (removed)

src/etc/inc/interfaces.inc:        if (!empty(trim(@file_get_contents("/tmp/{$realif}_router")))) {
src/etc/inc/interfaces.inc:        if (!empty(trim(@file_get_contents("/tmp/{$realifv6}_routerv6")))) {

Describe the solution you like

Remove manual spots for file write and maybe even file read.

Describe alternatives you considered

Need to get this done to make the interface handling more robust.

Additional context

#5862

[fichtner]

Just ditch all of it as evidence suggests it isn't used anywhere except pppoe and dhclient where it may have been MIA since d7b3d61 anyway.