FramesData: Don't leak allocated frames, and remove unused functions

Valgrind report:

```
14,960 (11,544 direct, 3,416 indirect) bytes in 37 blocks are definitely lost in loss record 20,483 of 20,540
   at 0x4837DEF: operator new(unsigned long) (vg_replace_malloc.c:334)
   by 0xC06963: rtengine::FramesData::FramesData(Glib::ustring const&, std::unique_ptr<rtengine::RawMetaDataLocation, std::default_delete<rtengine::RawMetaDataLocation> >, bool) (imagedata.cc:1121)
   by 0xBD774F: rtengine::DFManager::addFileInfo(Glib::ustring const&, bool) (dfmanager.cc:380)
   by 0xBD6E90: rtengine::DFManager::init(Glib::ustring) (dfmanager.cc:303)
   by 0xC3EC5D: rtengine::init(rtengine::Settings const*, Glib::ustring, Glib::ustring, bool) [clone ._omp_fn.0] (init.cc:93)
   by 0x897CABD: gomp_thread_start (team.c:120)
   by 0x89B7A9C: start_thread (in /usr/lib/libpthread-2.28.so)
   by 0x8ACCB22: clone (in /usr/lib/libc-2.28.so)
```

rtengine/imagedata.h

@@ -20,6 +20,7 @@
 #define __IMAGEDATA_H__
 
 #include <cstdio>
+#include <memory>
 #include "rawimage.h"
 #include <string>
 #include <glibmm.h>
@@ -89,7 +90,7 @@ public:
 class FramesData : public FramesMetaData {
 private:
     // frame's root IFD, can be a file root IFD or a SUB-IFD
-    std::vector<FrameData*> frames;
+    std::vector<std::unique_ptr<FrameData>> frames;
     // root IFD in the file
     std::vector<rtexif::TagDirectory*> roots;
     IptcData* iptc;
@@ -102,7 +103,6 @@ public:
     void setDCRawFrameCount (unsigned int frameCount);
     unsigned int getRootCount () const;
     unsigned int getFrameCount () const;
-    FrameData *getFrameData (unsigned int frame) const;
     bool getPixelShift () const;
     bool getHDR (unsigned int frame = 0) const;
     std::string getImageType (unsigned int frame) const;