Do not override AddonSettings.REQUIRE_SIGNING

Do not override AddonSettings.REQUIRE_SIGNING

This is not necessary, as it is only checked in
XPIDatabase.jsm::mustSign(), which we can override instead, and in
aboutaddonsCommon.js::isDisabledUnsigned(), which we cannot affect
anyways and only uses it to show a red warning bar for legacy addons.

This also removes the old AddonSettings.ALLOW_LEGACY_EXTENSIONS
override, which was only used in Extension.jsm::experimentsAllowed()
and XPIDatabase.jsm::isDisabledLegacy().

This is in preparation for the ESM-ification endeavours going on at
Mozilla, which will make importing a module's global object (as we did
with `let Xdb = Cu.import(...)`) impossible, but was needed to get a
reference to AddonSettings (instead of a copy). Step two will be
replacing Cu.import() with ChromeUtils.import(), which has stricter
semantics about exported objects (doesn't hurt us; see Bug 1766114).