build: Remove redundant mavenCentral repository declarations (#5638)

settings.gradle.kts already declares google(), mavenCentral() and mavenLocal()
via dependencyResolutionManagement for every project, so the module-level
repositories { mavenCentral() } blocks were redundant. With the default
PREFER_PROJECT mode they only narrowed each project to mavenCentral; removing
them falls back to the central superset and resolution is unaffected.