Bug 16285: Exclude ClearKey system for now

In the past the ClearKey system had not been compiled when specifying
--disable-eme. But that changed and it is even bundled nowadays (see:
Mozilla's bug 1300654). We don't want to ship it right now as the use
case for it is not really visible while the code had security
vulnerabilities in the past.

browser/installer/package-manifest.in

@@ -438,8 +438,8 @@ bin/libfreebl_64int_3.so
 #endif
 
 ; media
-@RESPATH@/gmp-clearkey/0.1/@DLL_PREFIX@clearkey@DLL_SUFFIX@
-@RESPATH@/gmp-clearkey/0.1/manifest.json
+;@RESPATH@/gmp-clearkey/0.1/@DLL_PREFIX@clearkey@DLL_SUFFIX@
+;@RESPATH@/gmp-clearkey/0.1/manifest.json
 
 #ifdef MOZ_DMD
 ; DMD