inspect-library: Remove the older JSON based output

Ludovico de Nittis requested to merge wip/denittis/line_based into main

In inspect-library we only care about the line based output.

With this commit we make the line based format the default and remove the older JSON based one. The option --line-based is still available in inspect-library to avoid any unexpected breakage and we expect to remove it after a couple of Steam Runtime releases. In inspect-library-libelf instead, we already removed the --line-based option because it is a new helper and there are no backwards-compat concerns.

Based on !540 (merged)

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