Remove ReentrantGuard

[He-Pin]

Motivation:
In Scala 2.x it will create a closure every time, that code is simple, if we can use it with Scala 3 's inline, then that's fine, I think we should just remove the code.

Should deprecate it first in 1.2.x

[pjfanning]

I am against this. No justification has been provided. The class is used in only a few places and and it will be hard to test if removing it causes concurrency issues.

https://github.com/search?q=repo%3Aapache%2Fpekko%20ReentrantGuard&type=code