env: common: Drop env_get_char_memory()

This function is the same as env_get_char_spec() apart from dropping the
brackets. Drop the brackets from env_get_char_spec() and use that instead
of env_get_char_memory().

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>