golang: Remove RSTRIP definition from golang-package.mk

Since RSTRIP is defined in rules.mk as a recursively expanded variable,
there is no need to define it again after setting STRIP in
golang-package.mk.

This also adds a note to the comment for GO_PKG_LDFLAGS to say that -s
and -w flags are not necessary.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>