Makefile: Remove 'quiet' variable

'quiet' variable is set and exported, but it is not used. We can safely
remove it.

This variable is inherited from the Makefile of the Linux kernel, and
is not used in Buildroot.

In support/scripts/mkmakefile, 'quiet' value is checked, but the test
is always true ('quiet' is never set to silent_), so the test can be
removed as well.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Reviewed-by: "James Knight" <james.d.knight@live.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>