classes/terminal: fix pseudo exiting when launching devshell

In dylan, since the entire bitbake process is run under pseudo,
LD_PRELOAD is set when we collect BB_ORIGENV and thus when we construct
the devshell environment from the latter, LD_PRELOAD is included.
However, for a fakeroot task we explicitly run the devshell under pseudo
(e.g. "pseudo /bin/bash"), and if LD_PRELOAD is set to preload
libpseudo.so when pseudo is run, it seems to exit immediately without
error. Since LD_PRELOAD shouldn't be exported anyway, exclude this from
the environment so it doesn't prevent running the shell.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Tested-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>