reverseproxy: Remove circuitbreaker module (see #3331)

Moving to https://github.com/caddyserver/circuitbreaker

Nobody was using it anyway -- it works well, but something got fumbled
in a refactoring *months* ago. Turns out that we forgot the interface
guards AND botched a method name (my bad) - Ok() should have been OK().
So it would always have thrown a runtime panic if it tried to be loaded.
The module itself works well, but obviously nobody used it because
nobody reported the error. Fixing this while we move it to the new repo.

Removing this removes the last Bazaar/Launchpad dependency (I think).

modules/caddyhttp/reverseproxy/healthchecks.go

@@ -113,7 +113,8 @@ type PassiveHealthChecks struct {
 
 // CircuitBreaker is a type that can act as an early-warning
 // system for the health checker when backends are getting
-// overloaded.
+// overloaded. This interface is still experimental and is
+// subject to change.
 type CircuitBreaker interface {
 	OK() bool
 	RecordMetric(statusCode int, latency time.Duration)