pdfjs.disabled used to be part of RFP until Bug 1838415; lock pref to false in stable

in FF116 1838415 in Don't spoof explicitly disabled pdfJS RFP no longer ignores the pref pdfjs.disabled. The original RFP protection was done to help against disk leaks and to provide a uniform fingerprint [1]

Given these two aspects, we should just lock the pref in stable release

[1] There are only two possible values for combined plugins, mimeTypes and pdfViewerEnabled