Remove unnecessary variable ypipe_conflate_t::reader_awake

[alokpr]

This variable is not necessary because the only possible value is "false". This variable also had data race reported by TSAN.

Fixes zeromq/libzmq#4369

[alokpr]

@bluca can you please review the PR?

[mitchellwrosen]

This change looks good to me, however the comment above flush probably needs tweaking as well:

    //  Flush is no-op for conflate ypipe. Reader asleep behaviour
    //  is as of the usual ypipe.
    //  Returns false if the reader thread is sleeping. In that case,
    //  caller is obliged to wake the reader up before using the pipe again.

[bluca]

This variable also had data race reported by TSAN.

TSAN does not understand the complicated pipes sharing that is implemented here, I suggest to simply avoid running it