removeOverrides() evicts the entire cache

[albertodebortoli]

As @dchakarov pointed out, removeOverrides evicts the entire cache but it could just remove the values that have been overridden.

I think this should be a simple fix.

From:

public func removeOverrides() -> Set<Variable> {
    queue.sync(flags: .barrier) {
        defer { cache.evict() }
        ...

to:

public func removeOverrides() -> Set<Variable> {
    queue.sync(flags: .barrier) {
        guard let mutableValueProvider = mutableValueProvider else { return  [] }
        let variables = mutableValueProvider.variables
        for variable in variables {
            cache.removeValue(forKey: variable)
            DispatchQueue.main.async {
                self.subjectsRefs[variable]?.send(completion: .finished)
                self.subjectsRefs[variable] = nil
            }
        }
        ...