According to some internal communication, these two functions are place
holders for future enhancements. Because they use a variable number of
parameters, the implementation defined a reserved keyword for them in the
parser grammar.

Unfortunately, doing so creates a bug similar to Bug 21114 reported for the
function FORMAT.

In the 5.1 code base, due to improvements in the code implemented with bug
21114, having a reserved keyword for functions with a variable number of
arguments is not needed any more by the implementation.

As a result, this fix removes the place-holder implementation, and removes
the unnecessary reserved keywords. Should the functions UNIQUE_USERS and
GROUP_UNIQUE_USERS be finally implemented in a later release, the
implementation should sub class Create_native_func in sql/item_create.cc.
For example, see the class Create_func_concat.