I'm afraid there is no easy way to decide, but 99% of the time it's pretty logical.
For example with BOB the first title is 1:13:08, which logically makes it the main part. You'll want to keep this otherwise there'll be no film
Next is 51:57, which is some sort of extra. Click on the preview tab and see what it is....if you want to keep it then do do, but in this case you are trying to cut down on space and it's the only big option to remove
Lastly is 0:0, which doesn't matter whether you remove it or not, but i still would.
Rule of thumb = longest title is main movie, then the extras are next, then the 1 - 10 second titles are normally FBI warnings. By clicking on the preview tab you can see.
As this is a series, you will have to see what the 51:57 is. If it's episode 2 or whatever then you will need to leave this in, otherwise remove it.