mips: Remove unused private members to fix compile-time warning

Certain versions of clang complain about unused private members if
they are not used. This changeset removes such members from the
MIPS-specific classes to silence the warning.

src/dev/mips/malta_io.hh

@@ -51,9 +51,6 @@
  */
 class MaltaIO : public BasicPioDevice
 {
-  private:
-    struct tm tm;
-
   protected:
 
     class RTC : public MC146818