revert: remove LibreSign SaaS-specific fragment config

[vitormattos]

Summary

• remove LibreSign/SaaS-specific fragment webhook environment variables from the base stack
• remove LibreSign-specific example env entries from the general-purpose repository
• remove the SaaS fragment upload nginx override from the shared wordpress-docker config

Why

wordpress-docker is a general-purpose repository and should not contain configuration that is specific to LibreSign or the SaaS composition. Those overrides now live in the saas repository local override layer.

Validation

• local SaaS stack still responds with HTTP 200 on http://localhost
• the shared header/footer fragment flow continues to work through the SaaS-owned override config