Remove stale Microsoft.DBforPostgreSQL/PostgreSQL/ ARM lease (case-collision fix) (Azure#43889)

Removes .github/arm-leases/postgresql/Microsoft.DBforPostgreSQL/PostgreSQL/lease.yaml
which case-collides with the sibling PostgreSql/lease.yaml on Windows/macOS
filesystems and is no longer referenced by the spec layout. The lowercase
PostgreSql/ folder matches the service folder casing introduced by
Azure/azure-rest-api-specs-pr#28858 / Azure#43777
(postgresql folder restructure), so keeping PostgreSql/ preserves lease
coverage for that PR while eliminating the case-only duplicate.

Supersedes Azure#43887, which removed the lowercase variant; removing the lowercase
one would re-trigger ARMModelingReviewRequired on the partner restructure PR
because the lease checker looks up the exact-cased path.

Co-authored-by: Vikeshi Tiwari <vikestiwari@microsoft.com>

Author: 2 people