[Firefox addon] Remove the unused `MOZ_CENTRAL` constant

The last (and only) usage of `MOZ_CENTRAL` was removed in PR 3036, so it's been unused for almost four years now.
If we need to have different code-paths for `FIREFOX`/`MOZCENTRAL` builds, the preprocessor should (and has) been used instead.

extensions/firefox/content/PdfStreamConverter.jsm

@@ -23,8 +23,7 @@ const Cc = Components.classes;
 const Ci = Components.interfaces;
 const Cr = Components.results;
 const Cu = Components.utils;
-// True only if this is the version of pdf.js that is included with firefox.
+
-const MOZ_CENTRAL = JSON.parse("PDFJSSCRIPT_MOZ_CENTRAL");
 const PDFJS_EVENT_ID = "pdf.js.message";
 const PDF_CONTENT_TYPE = "application/pdf";
 const PREF_PREFIX = "PDFJSSCRIPT_PREF_PREFIX";

make.js

@@ -397,8 +397,6 @@ target.firefox = function() {
       FIREFOX_BUILD_CONTENT_DIR + 'PdfStreamConverter.jsm');
   sed('-i', /PDFJSSCRIPT_PREF_PREFIX/, FIREFOX_PREF_PREFIX,
       FIREFOX_BUILD_CONTENT_DIR + 'PdfStreamConverter.jsm');
-  sed('-i', /PDFJSSCRIPT_MOZ_CENTRAL/, 'false',
-      FIREFOX_BUILD_CONTENT_DIR + 'PdfStreamConverter.jsm');
   sed('-i', /PDFJSSCRIPT_PREF_PREFIX/, FIREFOX_PREF_PREFIX,
       FIREFOX_BUILD_CONTENT_DIR + 'PdfjsChromeUtils.jsm');
 
@@ -451,8 +449,6 @@ target.mozcentral = function() {
       MOZCENTRAL_CONTENT_DIR + 'PdfStreamConverter.jsm');
   sed('-i', /PDFJSSCRIPT_PREF_PREFIX/, MOZCENTRAL_PREF_PREFIX,
       MOZCENTRAL_CONTENT_DIR + 'PdfStreamConverter.jsm');
-  sed('-i', /PDFJSSCRIPT_MOZ_CENTRAL/, 'true',
-      MOZCENTRAL_CONTENT_DIR + 'PdfStreamConverter.jsm');
   sed('-i', /PDFJSSCRIPT_PREF_PREFIX/, MOZCENTRAL_PREF_PREFIX,
       MOZCENTRAL_CONTENT_DIR + 'PdfjsChromeUtils.jsm');
 };