Since it is declared as a string AVOption, the generic freeing code
attempts to free it on codec close. Some codecs might have already freed
it elsewhere (or didn't even allocate it with av_malloc() in the first
place), so this might lead to an invalid free.

There is no point in having this field accessible as an AVOption, so
remove it from the options table.

Fixes Bug 380.

CC: libav-stable@libav.org