utils: drop exported var env caching for oe_popen

utils: drop exported var env caching for oe_popen

This was dangerous, as an oe_run executed from the configuration metadata
would result in caching the config metadata information, rather than the final
metadata information.  We could also cache it by the datasmart instance, but I
don't think its worth the trouble at this time, particularly given the next
bitbake release exports all exported vars into the process environment
automatically, avoiding the need for any of this.

Signed-off-by: Chris Larson <chris_larson@mentor.com>