Mark deprected methods and remove internal usage

This does not put the deprecated marker on methods that will or could resolve
to the same overload, e.g.:

void foo(bool bar = true); // <-- not marked
void foo(Bar bar) // <-- since this will have a default argument in the new version

taglib/mpc/mpcfile.h

@@ -198,7 +198,7 @@ namespace TagLib {
        * \deprecated
        * \see strip
        */
-      void remove(int tags = AllTags);
+      TAGLIB_DEPRECATED void remove(int tags = AllTags);
 
       /*!
        * Returns whether or not the file on disk actually has an ID3v1 tag.

taglib/mpc/mpcproperties.h

@@ -74,7 +74,7 @@ namespace TagLib {
        *
        * \deprecated
        */
-      virtual int length() const;
+      TAGLIB_DEPRECATED virtual int length() const;
 
       /*!
        * Returns the length of the file in seconds.  The length is rounded down to