avconv: use read_file() for reading the 2pass stats

Also, drop the now unused cmdutils_read_file(). There is no reason to
have two functions doing essentially the same thing.

cmdutils.h

@@ -460,18 +460,6 @@ int show_sample_fmts(void *optctx, const char *opt, const char *arg);
  */
 int read_yesno(void);
 
-/**
- * Read the file with name filename, and put its content in a newly
- * allocated 0-terminated buffer.
- *
- * @param filename file to read from
- * @param bufptr location where pointer to buffer is returned
- * @param size   location where size of buffer is returned
- * @return 0 in case of success, a negative value corresponding to an
- * AVERROR error code in case of failure.
- */
-int cmdutils_read_file(const char *filename, char **bufptr, size_t *size);
-
 typedef struct PtsCorrectionContext {
     int64_t num_faulty_pts; /// Number of incorrect PTS values so far
     int64_t num_faulty_dts; /// Number of incorrect DTS values so far