main/boost1.75: don't use custom statx implementation

When there is no statx function, but the statx syscall is detected, boost-filesystem implements a statx function that directly calls the syscall.

This is broken, because statx might not be implemented on the running system, and there is no logic to fall back to fstatat.

Remove the build-time detection of the statx syscall, so that the code falls back to stat.

Fixes #12349 (closed)