[subtitles] Don't search for external subtitles when opening a LiveTv input stream

[ace20022]

This commit also modifies the check from 6 ifs to 1 if with a disjunctive condition and removes a commented line.

vdrfan@ pointed me to this at PR #1337.

[opdenkamp]

change itself looks fine, but could you split out the cosmetics into a separate commit

[ace20022]

Two more things:

• What about isMythTv() and isTuxBox(), and maybe others?
• The wiki entry http://wiki.xbmc.org/index.php?title=Subtitles#Compressed_subtitles states that the name of the archive has to be the name of the video file. This is not the case, any archive in the search path is examined. Furthermore, besides rar also zip is supported and there is no option to "automatically turn on packed subtitles".