Disable regress-crbug-518748. It is failing/flaking on many bots

BUG=chromium:518748
TBR=mstarzinger@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1306443002 .

Cr-Commit-Position: refs/heads/master@{#30256}

test/mjsunit/mjsunit.status

@@ -258,7 +258,7 @@
   'regress/regress-4279': [PASS, ['isolates', SKIP]],
 
   # BUG(chromium:518748)
-  'regress/regress-crbug-518748': [PASS, ['mode == debug or arch == arm', SKIP]],
+  'regress/regress-crbug-518748': [SKIP],
 }],  # ALWAYS
 
 ['novfp3 == True', {