Merge bitcoin/bitcoin#29865: util: remove unused cpp-subprocess options

13adbf7 remove unneeded environment option from cpp-subprocess (Sebastian Falbesoner)
2088777 remove unneeded cwd option from cpp-subprocess (Sebastian Falbesoner)
03ffb09 remove unneeded bufsize option from cpp-subprocess (Sebastian Falbesoner)
79c3036 remove unneeded close_fds option from cpp-subprocess (Sebastian Falbesoner)
62db8f8 remove unneeded session_leader option from cpp-subprocess (Sebastian Falbesoner)
80d008c remove unneeded defer_spawn option from cpp-subprocess (Sebastian Falbesoner)
cececad remove unneeded preexec function option from cpp-subprocess (Sebastian Falbesoner)
633e45b remove unneeded shell option from cpp-subprocess (Sebastian Falbesoner)

Pull request description:

  The newly introduced cpp-subprocess library provides a good number of options for the `Popen` class:
  https://github.com/bitcoin/bitcoin/blob/0de63b8b46eff5cda85b4950062703324ba65a80/src/util/subprocess.hpp#L1009-L1020
  Some of them are either not fully implemented (`shell`, missing an implementation on Windows), implemented in an ugly way (e.g. using "Impoverished, meager, needy, truly needy version of type erasure" for `preexec_func` according to the author's own words) or simply unlikely to be ever needed for our external signer use-case (`defer_spawn`). Instead of maintaining incomplete and/or unneeded code, I'd suggest to get rid of it and only keep support for options if there is a strong reason for it.

ACKs for top commit:
  achow101:
    ACK 13adbf7
  hebasto:
    re-ACK 13adbf7.

Tree-SHA512: 8270da27891cb659da2ef6062a23f4b86331859b15ac27b79ae7433b14f5bd7efaba621f2b3ba1953708d0f38377a8bd23ef1cc0f28b9c152ac8958dd9eec6b0

Author: achow101