builtin/gc: drop redundant local variable

builtin/gc: drop redundant local variable

We have two different variables that track the quietness for git-gc(1):

  - The local variable `quiet`, which we wire up.

  - The `quiet` field of `struct maintenance_run_opts`.

This leads to confusion which of these variables should be used and what
the respective effect is.

Simplify this logic by dropping the local variable in favor of the
options field.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>