Remove support for old, uncompressed backtrace fmt

lib/sidekiq/api.rb

@@ -433,20 +433,9 @@ module Sidekiq
     end
 
     def uncompress_backtrace(backtrace)
-      if backtrace.is_a?(Array)
+      decoded = Base64.decode64(backtrace)
-        # Handle old jobs with raw Array backtrace format
+      uncompressed = Zlib::Inflate.inflate(decoded)
-        backtrace
+      Sidekiq.load_json(uncompressed)
-      else
-        decoded = Base64.decode64(backtrace)
-        uncompressed = Zlib::Inflate.inflate(decoded)
-        begin
-          Sidekiq.load_json(uncompressed)
-        rescue
-          # Handle old jobs with marshalled backtrace format
-          # TODO Remove in 7.x
-          Marshal.load(uncompressed)
-        end
-      end
     end
   end
 

test/test_api.rb

@@ -274,27 +274,6 @@ describe "API" do
       assert_nil job.enqueued_at
     end
 
-    it 'handles previous (raw Array) error_backtrace format' do
-      add_retry
-      job = Sidekiq::RetrySet.new.first
-      assert_equal ["line1", "line2"], job.error_backtrace
-    end
-
-    it "handles previous (marshalled Array) error_backtrace format" do
-      backtrace = ["line1", "line2"]
-      serialized = Marshal.dump(backtrace)
-      compressed = Zlib::Deflate.deflate(serialized)
-      encoded = Base64.encode64(compressed)
-
-      payload = Sidekiq.dump_json("class" => "ApiWorker", "args" => [1], "queue" => "default", "jid" => "jid", "error_backtrace" => encoded)
-      Sidekiq.redis do |conn|
-        conn.zadd("retry", Time.now.to_f.to_s, payload)
-      end
-
-      job = Sidekiq::RetrySet.new.first
-      assert_equal backtrace, job.error_backtrace
-    end
-
     describe "Rails unwrapping" do
       SERIALIZED_JOBS = {
         "5.x" => [