rules: drop -Wno-error additional flags from default TARGET_CFLAGS

rules: drop -Wno-error additional flags from default TARGET_CFLAGS

We currently enable -Wno-error=unused-but-set-variable and
-Wno-error=unused-result by default on every compile package.

While this is (relatively) unharmful, we should follow other project
direction and starts enforcing good code quality. For example the linux
kernel recently started to enforce Wall by default and clean code is
mandatory for inclusion.

Drop for good these flags and and make it mandatory to correctly handle
return values at least with a warning log if they are not strictly error
condition.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>