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>

src/libsysprof-capture/sysprof-collector.c

@@ -316,8 +316,6 @@ request_writer (void)
       if (fdstr != NULL)
         peer_fd = atoi (fdstr);
 
-      unsetenv ("SYSPROF_CONTROL_FD");
-
       if (peer_fd > 0)
         {
           (void) set_fd_blocking (peer_fd);