remove old Gem repo to make way for the stop-gap repo from git.purrdata.net

.gitmodules

-[submodule "Gem"]
-        path = Gem
-        url = git://git.code.sf.net/p/pd-gem/gem
 [submodule "l2ork_addons/rtcmix-in-pd"]
 	path = externals/rtcmix-in-pd
 	url = https://github.com/jwmatthys/rtcmix-in-pd.git

Gem @ 374f7136

-Subproject commit 374f71368f4a54e8d0f7887460a178e9020a1ba4