FIX: remove hiredis gem which is no longer needed

Previously some local micro-benchmarks revealed it was not giving any perf
benefits.

Now that we upgraded to 2.6.5 we are seeing some segfaults.

No need to carry this dependency around anymore.

We can re-evaluate in future if it improves perf and fix the segfaults.

Gemfile

@@ -36,8 +36,6 @@ gem 'mail', require: false
 gem 'mini_mime'
 gem 'mini_suffix'
 
-gem 'hiredis'
-
 # holding off redis upgrade temporarily as it is having issues with our current
 # freedom patch, we will follow this up.
 #
@@ -45,7 +43,7 @@ gem 'hiredis'
 # /var/www/discourse/vendor/bundle/ruby/2.5.0/gems/redis-4.1.0/lib/redis/client.rb:93:in `delete'
 # /var/www/discourse/vendor/bundle/ruby/2.5.0/gems/redis-4.1.0/lib/redis/client.rb:93:in `initialize'
 # /var/www/discourse/lib/freedom_patches/redis.rb:7:in `initialize'
-gem 'redis', '4.0.1', require:  ["redis", "redis/connection/hiredis"]
+gem 'redis', '4.0.1'
 gem 'redis-namespace'
 
 gem 'active_model_serializers', '~> 0.8.3'

Gemfile.lock

@@ -143,7 +143,6 @@ GEM
     hashdiff (0.3.9)
     hashie (3.6.0)
     highline (1.7.10)
-    hiredis (0.6.3)
     hkdf (0.3.0)
     htmlentities (4.3.4)
     http_accept_language (2.0.5)
@@ -467,7 +466,6 @@ DEPENDENCIES
   flamegraph
   gc_tracer
   highline (~> 1.7.0)
-  hiredis
   htmlentities
   http_accept_language (~> 2.0.5)
   listen