bitbake.conf: Drop unexports from a different era

bitbake.conf: Drop unexports from a different era

A long time ago, we used to pass our parent execution environment into our task
environments during build. We stopped doing that for reproducibility and consistency
of builds. The variables TARGET_ARCH, DISTRO and MACHINE are not exported into
tasks and hence we don't need to unexport them. The resasons these exist is
therefore no longer relavent and they can be removed. This happens to improve
bitbake -e output from a user commandline perspective.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>