lavf/srtdec: trim line break event separators from packet.

The muxer add them automatically, so this avoid having a bunch of line
breaks all over the output files. One '\n' is still kept/added because
the lavc subrip decoder seems to have trouble with line ending abruptly
(it doesn't flush correctly the tags). This bug is harmless but should
be fixed; though, this doesn't look like a trivial change. When this bug
gets fixed, we can consider removing the '\n' at the end of the packet.

The 2048B buffer limit was also removed while moving to AVBPrint API.
Note that this doesn't really matter since the decoder is limited as
well.
