Remove observer gem

This was only necessary when using factory_girl, but we've now updated
to factory_bot.

Author: eugeneius

Gemfile

@@ -10,6 +10,4 @@ if RUBY_VERSION < "2.2"
 elsif RUBY_VERSION < "3.1"
   gem "sqlite3", "~> 1.4.0"
   gem "term-ansicolor", "< 1.10.3"
-elsif RUBY_VERSION >= "3.4"
-  gem "observer"
 end