[Doc] Remove admonition about testing from MixtureFugacityTP

The classes derived from this (RedlichKwongMFTP and PengRobinson)
have adequate tests.

include/cantera/thermo/MixtureFugacityTP.h

@@ -43,14 +43,6 @@ namespace Cantera
  * mixture is also allowed. The main subclass is currently a mixture Redlich-
  * Kwong class.
  *
- * @attention This class currently does not have any test cases or examples. Its
- *     implementation may be incomplete, and future changes to %Cantera may
- *     unexpectedly cause this class to stop working. If you use this class,
- *     please consider contributing examples or test cases. In the absence of
- *     new tests or examples, this class may be deprecated and removed in a
- *     future version of  %Cantera. See
- *     https://github.com/Cantera/cantera/issues/267 for additional information.
- *
  * Several concepts are introduced. The first concept is there are temporary
  * variables for holding the species standard state values of Cp, H, S, G, and V
  * at the last temperature and pressure called. These functions are not