Stop using mozPrintCallback in generic builds

[brendandahl]

Over in https://bugzilla.mozilla.org/show_bug.cgi?id=1524640 the fix is only allowed for the built in pdf.js viewer, so if fonts are cleaned before the callback, printing will break. There's also been a little discussion on moving mozPrintCallback to only be allowed by the internal PDF.js viewer since no other browser implement it nor do they plan to.

[Snuffleupagus]

The generic printing code, which is found in web/pdf_print_service.js, hasn't been using mozPrintCallback ever since PR #7697 :-)

The only usage of mozPrintCallback is found in https://github.com/mozilla/pdf.js/blob/master/web/firefox_print_service.js, a file which is only bundled into MOZCENTRAL builds:

pdf.js/web/viewer.js

Lines 44 to 47 in 7d24fc3

	if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('FIREFOX || MOZCENTRAL')) {
	require('./firefoxcom.js');
	require('./firefox_print_service.js');
	}

[brendandahl]

Hah, missed that one.