Remove puma_worker_killer since it's unused

Gemfile

@@ -8,7 +8,6 @@ gem 'jbuilder', '~> 2.10'
 gem 'bootsnap', '~> 1.4', require: false # Large rails application booting enhancer
 gem 'font_assets', '~> 0.1.14' # for serving fonts on cdn https://github.com/ericallam/font_assets
 gem 'hamster', '~> 3.0' # Thread-safe collection classes for Ruby
-gem 'puma_worker_killer', '~> 0.1.1' # TODO: Investigate why puma workers need to be killed.
 gem 'puma', '~> 4.0', '>= 4.0.1'
 gem 'rake', '~> 12.3.2'
 gem 'sassc-rails', '~> 2.1', '>= 2.1.2'

Gemfile.lock

@@ -158,8 +158,6 @@ GEM
       rack
     foreman (0.87.1)
     geocoder (1.6.3)
-    get_process_mem (0.2.4)
-      ffi (~> 1.0)
     globalid (0.4.2)
       activesupport (>= 4.2.0)
     hamster (3.0.0)
@@ -241,9 +239,6 @@ GEM
     public_suffix (3.1.1)
     puma (4.3.5)
       nio4r (~> 2.0)
-    puma_worker_killer (0.1.1)
-      get_process_mem (~> 0.2)
-      puma (>= 2.7, < 5)
     rack (2.2.3)
     rack-attack (5.4.2)
       rack (>= 1.0, < 3)
@@ -448,7 +443,6 @@ DEPENDENCIES
   pry (~> 0.12.2)
   pry-byebug (~> 3.7.0)
   puma (~> 4.0, >= 4.0.1)
-  puma_worker_killer (~> 0.1.1)
   qx!
   rack-attack (~> 5.2)
   rack-ssl (~> 1.4)