perl-version: remove PERL* assignments

perl-version: remove PERL* assignments

The perl-version.bbclass executes functions which can depend on
variables potentially populated by native, such as `libdir`.  The
sanity `native-last` suggests that recipes should `inherit native`
last, but when that is done the variables like PERLVERSION end up
as `None`, since `${STAGING_LIBDIR}` needs `${libdir}` which is not
yet populated (by native).

All recipes in poky and widely used meta-layers have already been
updated to use the functions directly instead of relying on these
problematic variables.  Delete the variables so that future recipes
do not make the mistake of using them.

Related: openbmc/openbmc#3770
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>