RTPDUMP is not RTPDUMP

When the encoder/decoder are in RTPDUMP/VoIP mode then no longer the rtpdump.h/rtpdump.c files are used which have been used by EVS. The alternative implementation in lib_util/ivas_rtp_file.c allows encoder/decoder round-trip but it is in fact not using the RTPDUMP format that is specified at https://www.cs.columbia.edu/~hgs/software/rtptools/ and which has been used previously in other 3GPP exercises. Using the real format would also be essential to check real-world packet captures. This has been noticed during spec writing and checking the respective code.

When looking at this also three bugs in reading rtpdump and mime_io introduced by SMALL_CODE_IMPROVEMENTS have been identified that went unnoticed - likely because the files were not in use.