Remove unneeded pragmas on Windows now that SSE2 is available.

Basically reverts the change made for bug 316740 because this is no
longer needed since windows requires SSE2 now.

The test in http://jsfiddle.net/gMKh9/ now takes about 1.90 sec on my
Z600, compared to the original report of 4+ sec. This is still
somewhat slower than a MacBook Pro (1.2 s) or Linux on a Z620 (1.5 s).

BUG=316740, 349320

Review URL: https://codereview.chromium.org/309763007

git-svn-id: svn://svn.chromium.org/blink/trunk@175394 bbb929c8-8fbe-4397-9dbb-9b2b20218538