cli_hush.c: remove broken sanity check

cli_hush.c: remove broken sanity check

This code is intended do prevent one from setting a shell variable abc
by doing

  abc=123

if an environment variable named abc already exists. However, the
check is broken, since the env_get is done before the split on =, so
we look up whether an environment variable "abc=123" exists, which is
obviously never the case.

One could move the code to below the split on =, but instead, just
remove it, saving a little .text: The check has never worked as
intended (it goes all the way back to the initial git commit), and it
would anyway not guard against one first setting the shell variable,
then doing 'env set abc xyz'.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>