Remove unnecessary require "rake"

This causes an error on bundle install when installing the gem from source.

16.66 [!] There was an error while loading `launchdarkly-server-sdk.gemspec`: cannot load such file -- rake. Bundler cannot continue.
16.66 
16.66  #  from /usr/local/bundle/ruby/3.3.0/bundler/gems/ruby-server-sdk-c7080edbad3a/launchdarkly-server-sdk.gemspec:6
16.66  #  -------------------------------------------
16.66  #  require "ldclient-rb/version"
16.66  >  require "rake"
16.66  #  
16.66  #  -------------------------------------------

launchdarkly-server-sdk.gemspec

@@ -3,7 +3,6 @@
 lib = File.expand_path("lib", __dir__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require "ldclient-rb/version"
-require "rake"
 
 # rubocop:disable Metrics/BlockLength
 Gem::Specification.new do |spec|