Revert "automatically disable signature requirement"

Revert "automatically disable signature requirement"

downgrading the warning color stopped working some time ago. for
completeness, as of nightly99 xpinstall.signatures.required is only
checked in these places, none of which affect us:
- nsContentSecurityUtils.cpp::DetectJsHacks()
  only for telemetry and relaxing some security measures we don't need
- BrowserGlue.jsm::_onWindowsRestored()
  gates call to _notifyUnsignedAddonsDisabled(), which isn't triggered
  by our stuff (signedState != SIGNEDSTATE_MISSING)
- AddonSettings.jsm::REQUIRE_SIGNING
  overridden by our config.js script, so doesn't matter
- XPIProvider.jsm::observe()
  calls XPIDatabase.updateAddonAppDisabledStates(), which calls
  this.updateAddonDisabledState(addon), which depends on
  this.isUsableAddon(aAddon), which returns true as long as we override
  this.isDisabledLegacy(aAddon)

This reverts commit 29dc0ab3a8f8bd6e781fb983cd076764480dce18.