Use this to replace the much-hated `g_debug()` which told people that
`posix_spawn()` (the fast path) wasn’t being used for various reasons.

If people want to make their process spawning faster now, they’ll have
to use a profiling tool like sysprof to check their program’s
performance. Shocking.

I think I was wrong to put this `g_debug()` in there in the first place
— it hasn’t served its purpose of making people speed up their spawn
paths to use `posix_spawn()`, it’s only cluttered up logs and frustrated
people.

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