malloc: remove !gd handling

Following the previous patch, malloc() is never called before gd is set,
so we can remove the special-case check for this condition.

This reverts commit 854d2b97 "dlmalloc: ensure gd is set for early
alloc".

Cc: Rabin Vincent <rabin@rab.in>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>