Bug 1800342: Make NS_NewPipe2 and NS_NewPipe infallible, to reflect reality. r=xpcom-reviewers,necko-reviewers,nika,kershaw

This patch doesn't change behavior; these APIs were already effectively
infallible, in the sense that it was impossible for them to return anything
other than NS_OK.

The actual API changes here are in xpcom/io/nsIPipe.idl and
xpcom/io/nsPipe3.cpp, and the rest of this patch is just updates to callsites,
removing code that handled failure-flavored return values from these functions.
(All of this removed code was already dead code, effectively.)

Differential Revision: https://phabricator.services.mozilla.com/D161947