specs: remove rspec_junit_formatter

It was unsed by CircleCI to generate test reports in the JUnit XML
format, but:

- We now use Github Actions, which has its own reporting system,
- It prevents us to upgrade to rspec > 3.

Gemfile

@@ -115,7 +115,6 @@ group :development, :test do
   gem 'graphql-schema_comparator'
   gem 'mina', git: 'https://github.com/mina-deploy/mina.git', require: false # Deploy
   gem 'pry-byebug' # Call 'byebug' anywhere in the code to stop execution and get a debugger console
-  gem 'rspec_junit_formatter', require: false
   gem 'rspec-rails'
   gem 'simple_xlsx_reader'
   gem 'spring' # Spring speeds up development by keeping your application running in the background

Gemfile.lock

@@ -588,8 +588,6 @@ GEM
       rspec-mocks (~> 3.10)
       rspec-support (~> 3.10)
     rspec-support (3.10.2)
-    rspec_junit_formatter (0.4.1)
-      rspec-core (>= 2, < 4, != 2.12.0)
     rubocop (1.10.0)
       parallel (~> 1.10)
       parser (>= 3.0.0.0)
@@ -855,7 +853,6 @@ DEPENDENCIES
   rgeo-geojson
   rqrcode
   rspec-rails
-  rspec_junit_formatter
   rubocop
   rubocop-rails_config
   rubocop-rspec-focused