Update portable thread pool event enablement checks

[kouvel]

• The original intent was to check only IsEnabled() on the caller side of the event method since it was seen that in very hot paths where events may be fired, more complicated checks involving keyword and verbosity increased the code size in the caller unnecessarily, making it slower. IsEnabled() (without keyword and verbosity checks) is a simple check that can be inlined and is less intrusive to the hot path callers. Instead, the callee (the public method in the event source) would check enablement based on keyword and verbosity, and in the really hot paths where it was necessary the callee was also attributed with NoInlining.
• From the comment regarding IsEnabled checks in Migrate coreclr's worker thread pool to be able to use the portable thread pool in opt-in fashion #38225 (comment), I understood that the common paths in the eventing infra would do the necessary checks including keyword and verbosity checks before firing an event. That does not seem to be the case, hence Make PortableThreadPool to check events correctly before firing them #45666.
• So I have reverted the part of e8043ff relating to IsEnabled checks, which was in response to the comment above, and reverted Make PortableThreadPool to check events correctly before firing them #45666 to restore the original intention

[sywhang]

LGTM. Thanks!