native: Drop special variable handling

Back when it was merged in https://git.yoctoproject.org/poky/commit/?id=c4257ed8b1040a5a0e9a95846d81961741239116


it appeared special handling in setVar was necessary. Likely due to
other fixes and improvements it no longer appears to be. The results of
"bitbake world -g" before and after this change are identical for oe-core
and meta-openembedded. Therefore drop the "parsing=True" special parameter to
setVar() meaning overrides can now be cleared.

This means that something like:

DEPENDS:append = " bash"

now functions as expected, adding bash to the target recipe and bash-native to
the native BBCLASSEXTENDED version.

Without this change, it would add bash to the native version as well as
bash-native which is clearly incorrect.

[YOCTO #15010]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 99a5105f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>