no bug - docs: remove references to non existing docs r=geckoview-reviewers,owlish

Depends on D194633

Differential Revision: https://phabricator.services.mozilla.com/D194634

browser/docs/index.rst

@@ -10,7 +10,6 @@ This is the nascent documentation of the Firefox front-end code.
    urlbar/index
    BrowserUsageTelemetry
    CommandLineParameters
-   app/pbproxy/docs/index
    components/enterprisepolicies/docs/index
    extensions/formautofill/docs/index
    components/newtab/docs/index
@@ -22,7 +21,6 @@ This is the nascent documentation of the Firefox front-end code.
    places/index
    components/newtab/content-src/asrouter/docs/index
    search/index
-   base/sslerrorreport/index
    base/tabbrowser/index
    touchbar/index
    components/uitour/docs/index