libsysprof-capture: Don’t unset SYSPROF_CONTROL_FD

By design, it needs to be usable from multiple threads so they can each request a thread-local mapped ring buffer. This becomes particularly obvious if you want to send sysprof_collector_mark()s from two libraries (say, GLib and libsoup); the first thread/library to call sysprof_collector_mark() will win.

Signed-off-by: Philip Withnall withnall@endlessm.com