Remove duplicated serverIdentity field

[TristanCacqueray]

This change fixes this compilation issue:

src/Simplex/Messaging/Notifications/Server/Env.hs:70:5: error:
    Multiple declarations of ‘serverIdentity’
    Declared at: src/Simplex/Messaging/Notifications/Server/Env.hs:68:5
                 src/Simplex/Messaging/Notifications/Server/Env.hs:70:5
   |
70 |     serverIdentity :: C.KeyHash
   |     ^^^^^^^^^^^^^^