FundamentalType: Drop KWSYS_CAN_CONVERT_UI64_TO_DOUBLE macro

This was used only for an old VS 6 version that did not support
conversion of `unsigned __int64` to `double`.  Support for this compiler
has been dropped from KWSys clients that used the macro.

Change-Id: Ia61beaa0251fc2b6d77632af3e610c08f64d11eb