Drop merged GraphQL schema for split files

Relay no longer needs a single merged schema.graphql: each project in
relay.config.json now reads the split graphql/*.graphql files directly
via `schema` (base.graphql) plus `schemaExtensions`. gqlgen already
consumed the split files, so the merge step only fed Relay and the
optional IDE GraphQL extension.

Remove the merge machinery (contrib/merge-graphql-schema.sh, the
RELAY_SCHEMAS make rules, and the gitignore entry) and drop the
graphql-config files (apps/trust/graphql.config.yml and the root
package.json graphql field); the Relay extension provides schema-aware
language features from relay.config.json on its own.

relay-compiler keeps generated artifacts in sync (stale ones are
removed automatically), so the relay npm script just runs the local
relay-compiler and the make target delegates to it.

Signed-off-by: Émile Ré <emile@probo.com>

.gitignore

@@ -21,7 +21,6 @@ compose/keycloak/probo-realm.json
 # Generated files (codegen)
 __generated__/
 pkg/server/api/*/v1/types/types.go
-pkg/server/api/*/v1/schema.graphql
 cfg/dev_local.yaml
 cfg/dev.yaml
 .env