Remove unused sqlite3 gem

Oh huh, I guess we used to use this for automated testing, but since
then I've moved the test database to just be in MySQL like everything
else, so I think we don't need this adapter anymore! Goodbye!

Gemfile

@@ -85,8 +85,3 @@ gem "sentry-rails", "~> 5.12"
 
 # For tasks that use shell commands.
 gem "shell", "~> 0.8.1"
-
-# For automated testing.
-group :test do
-	gem 'sqlite3', '~> 1.7'
-end

Gemfile.lock

@@ -381,8 +381,6 @@ GEM
       actionpack (>= 5.2)
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
-    sqlite3 (1.7.2)
-      mini_portile2 (~> 2.8.0)
     stackprof (0.2.26)
     stringio (3.1.0)
     swd (2.0.3)
@@ -466,7 +464,6 @@ DEPENDENCIES
   sentry-ruby (~> 5.12)
   shell (~> 0.8.1)
   sprockets (~> 4.2)
-  sqlite3 (~> 1.7)
   stackprof (~> 0.2.25)
   terser (~> 1.1, >= 1.1.17)
   thread-local (~> 1.1)