Don't print to stdout on NoGC GC request output

[chrisseaton]

This warning makes testing hard for me, because it's always printed, right on stdout. Can it be a proper logging line instead, disabled at a level when it doesn't appear by default? Or not included at all because it's obvious that NoGC won't do a GC.

mmtk-core/src/plan/nogc/global.rs

Line 83 in 76131c4

println!("Warning: User attempted a collection request, but it is not supported in NoGC. The request is ignored.");

CC @wks.

[qinsoon]

It should be a warn level log. I will fix that.