gpib: drop obscure "total number of samples" option

The gpib decoder tried to "flush" input data at a user specified sample
number when the input data lacked the respective edge which triggers the
processing during regular operation.

This is rather obscure a feature, not seen in any other decoder, perhaps
a workaround for bug #292, rather unaccessible to users (units of sample
numbers not times nor automatic detection of the EOF condition), highly
confusing according to user reports, and not covered by existing tests.

The mere presence of this option caused severe issues in application
code (see bug #1444). While there is no apparent fix that won't affect
other decoders. So let's drop this questionable feature. Valid and
complete captures should contain all relevent edges and thus decode
properly.