elm theme - don't use empty env vars eina_prefix handles anyway

elm theme - don't use empty env vars eina_prefix handles anyway

in 14ec825aa9ed9c6ede79d1729fdb291285b3bd6d cedric added trying
ELM_DATA_DIR env var explicitly for looking for themes. first if this
is NULL then we look in a bogus directory (null). secondly eina_prefix
already takes care of env vars like thsi as part of figuring out
prefix with correct namespace etc. so this just is a bug as opposed to
anything useful and the CORRECt implementation is already done in
eina_prefix.

@fix