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>

apps/trust/graphql.config.yml

@@ -1,5 +0,0 @@
-schema: '../../pkg/server/api/trust/v1/schema.graphql'
-extensions:
-  endpoints:
-    TrustCenter:
-      url: http://localhost:8080/api/trust/v1/graphql