tt(DELETEMACRO) removes a definition of a macro that was defined by
tt(DEFINEMACRO). This function takes one argument: the macro name to remove.

There is no error condition (except for syntax errors): when no macro with a
matching name was previously defined, no action is taken.

For example, the safe way to define a macro is by first undefining it. This
ensures that possible previous definitions are removed first:

    Example:
        verb(\
    DELETEMACRO(mymacro)
        )

