Fix -Wunused-private-field warnings

Resolved following issues when using *Clang* with `-Wall`:

* catch_console_colour private field `m_impl` only used on Windows when no ANSI

* catch_reporter_junit private field `m_currentTestSuccess` is not used at all

```
In file included from t.cpp:3:
In file included from ../lib/catch/include/catch.hpp:39:
In file included from ../lib/catch/include/internal/catch_impl.hpp:23:
In file included from ../lib/catch/include/internal/catch_console_colour_impl.hpp:11:
../lib/catch/include/internal/catch_console_colour.hpp:39:28: warning: private field 'm_impl' is not
      used [-Wunused-private-field]
        ConsoleColourImpl* m_impl;
                           ^
In file included from t.cpp:3:
In file included from ../lib/catch/include/catch.hpp:39:
In file included from ../lib/catch/include/internal/catch_impl.hpp:35:
../lib/catch/include/internal/../reporters/catch_reporter_junit.hpp:237:14: warning: private field
      'm_currentTestSuccess' is not used [-Wunused-private-field]
        bool m_currentTestSuccess;
```