Remove unnecessary aligned memory check when __SSE__ is used

Since the land of https://pdfium.googlesource.com/pdfium/+/3522876d5291922ddc62bf1b70d02743b0850673, memory is assured to be 16 byte aligned. So no need to do this check.

Plus, the removed code was causing bug in M36: https://code.google.com/p/pdfium/issues/detail?id=27.

BUG=None
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/418563002