pmbootstrap -s: remove option

Remove in favor of similar, but more visible "--no-install" option for "pmbootstrap flasher" and "pmbootstrap export".

It seems that the option wasn't really used by anybody, so let's just remove it without a deprication period. If dear reader thinks otherwise after this is merged, and is not happy with "--no-install", please state your reasoning in a new issue, so we can discuss it.

Related: !1863 (merged)