[PLAT-6818] rm WriteBehindViewComputationCacheTest

This intermittently-failing test tests unsupported old engine
functionality, so it is being removed in order to keep the
UNIT_SLOW build green rather than attempting to fix it up.
Merge pull request #239 from OpenGamma/topic/PLAT-6818

[PLAT-6818] Remove WriteBehindViewComputationCacheTest