The USE_DL_PREFIX define is used only by dlmalloc shipped with
native_client code. Chromium does not support shipping nacl with
prefixed malloc so the define actually breaks NaCl support in crosswalk.
Get rid of the extra unnecessary define.
Change-Id: I875c2afc95c051cdfbb0985e43b457be74e93f82 Signed-off-by: Piotr Tworek <p.tworek@samsung.com>