roll: pigweed pw_thread: Simplify disabling join() function when not supported

Use a bool instead of a type to simplify disabling the join function and
fix a minor style issue (kUnusedType used constant style).

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/257913
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Original-Revision: 717e4f58092e947eba23afb8be099d2ccf1247c3

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 0891a72c042e3e..717e4f58092e94
Roll-Count: 1
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Change-Id: I3b8c220410e510a207249d9e783a673c167cf43a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/showcase/sense/+/258032
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