mp4: Get rid of find_standard_meta().

mp4: Get rid of find_standard_meta().

We don't need a dedicated function and data structure for that. Just
open-code the logic in create_ilst() and clean up this function a bit
while at it. Specifically:

* Call the loop variable "n" rather than "metaptr" since it is not
a pointer but an unsigned integer.

* Abort if we encounter a tag item name which is not one of the five
standard names. This can never occur because the origin of these
strings is the code in aac_afh.c which only passes standard names.

* Drop the integer return value, since the function can never
fail. Make it return the buffer pointer instead and get rid of the
corresponding parameter.