refactor: Remove a bunch of useless group code from Messenger.c

[JFreegman]

Everything the Messenger Group struct held is now taken care of by the
GC_Chat instance

---

This change is 

[ghost]

DeepCode's analysis on #e42604 found:

⚠️ 1 warning 1 minor issue.

👉 View analysis in DeepCode’s Dashboard

---

☺️ If you want to provide feedback on our bot, here is how to contact us.

[codecov]

Codecov Report

Merging #1604 into ngc will decrease coverage by 0.02%.
The diff coverage is 67.74%.

@@            Coverage Diff             @@
##              ngc    #1604      +/-   ##
==========================================
- Coverage   81.38%   81.35%   -0.03%     
==========================================
  Files          95       95              
  Lines       20410    20351      -59     
==========================================
- Hits        16611    16557      -54     
+ Misses       3799     3794       -5     

Impacted Files	Coverage Δ
toxcore/group_chats.c	68.54% <35.71%> (-0.32%)	⬇️
toxcore/Messenger.c	86.02% <94.11%> (-0.33%)	⬇️
toxcore/friend_connection.c	94.13% <0.00%> (-0.28%)	⬇️
toxcore/net_crypto.c	93.58% <0.00%> (-0.19%)	⬇️
toxcore/onion_client.c	96.42% <0.00%> (+0.57%)	⬆️
toxcore/group_moderation.c	75.67% <0.00%> (+1.01%)	⬆️
toxav/groupav.c	82.99% <0.00%> (+1.21%)	⬆️
toxcore/friend_requests.c	97.77% <0.00%> (+4.44%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update facf274...e426047. Read the comment docs.