fix: Remove STALE event

[fabriziodemaria]

The reasons behind this change:

• This is currently not working as expected, since in case of activateAndFetchAsync the STALE event is emitted even if it shouldn't
• According to this WIP Proposal, no event is expected prior to "initialize".
• The STALE event is not meant to indicate a "not-ready cache" anyway: it's meant be used only if the system detects a change of flag evaluation data (for the same evaluation context). From the same PR mentioned above, a new events will be introduced for context reconciliation: PROVIDER_CONTEXT_PENDING and PROVIDER_CONTEXT_CHANGED

Similar to the Swift Provider change: spotify/confidence-sdk-swift#78