Remove OverrideCustomCheckRepeatTime from PersistenceSettings and related test configuration

[warwickschroeder]

What this removes

PersistenceSettings.OverrideCustomCheckRepeatTime has had no reader since #4147 and this deletes it, along with the one acceptance test line that still set it. No custom check changes its repeat time, because none of them ever consulted this property.

• The dead property. PersistenceSettings is the base every persister derives from, so a public property there reads as part of the host-to-persister contract. This one meant nothing: every custom check that ships passes a literal interval to its CustomCheck base constructor, six in ServiceControl.Persistence.RavenDB, five in ServiceControl.Audit.Persistence.RavenDB and FileSystemBodyStorageCustomCheck in the EF persister.
• How it died. The property arrived in Move MultiInstance tests to RavenDB 5 #3720 with the only thing that ever read it, SagaAuditDestinationCustomCheck, which took settings.OverrideCustomCheckRepeatTime ?? TimeSpan.FromMinutes(15) as its interval. Simplify the primary instance SagaAudit data store #4147 deleted that check and one of the two test lines that set the override, and left the property and the other test line behind.
• The test line. When_remote_instance_is_not_reachable set the override to two seconds, so it read as though it had made the custom checks run every two seconds. It had not. The test keeps both of its disableHealthChecks cases and every assertion, which is what it was actually written to cover.
• One visible side effect. ConfigurationApi serialises the whole settings object into GET /api/configuration, so overrideCustomCheckRepeatTime currently ships to ServicePulse as a null field and disappears from that payload. Nothing asserts on the shape of that response and ServicePulse reads fields by name.

Tests

• ServiceControl.MultiInstance.AcceptanceTests: When_remote_instance_is_not_reachable loses the line but keeps both cases, so it still covers that an unreachable remote instance does not fail a search.