remove define triggering pre-v9 placement new

Remove define of __IBM_ALLOW_OVERRIDE_PLACEMENT_NEW.
There appears to be an aliasing issue with 'placement new' and xlC
optimization.  Addition of a compiler fence is needed to quell the
improper optimization.  It appears sufficient to add the compiler fence
BSLS_PERFORMANCEHINT_PLACEMENT_NEW_FENCE after use of 'placement new'
though some might prefer to add the compiler fence both before and
after use of 'placement new' when the identifier passed into
'placement new' is used instead of the 'placement new' return value