Remove exit handling for GBWT buffer size in autoindex

[jeizenga]

Changelog Entry

To be copied to the draft changelog by merger:

• vg autoindex won't enter infinite loop when there is not enough disk space to write a GBWT

Description

Probably resolves #4183.

GBWT no longer exits when has insufficient buffer size, so there is no longer a reason to handle exit codes on the autoindex size. Also, the existing exit handling doesn't differentiate between different codes, so there is a risk that it can thrash when encountering exits for another reason, such as insufficient disk space.