Remove unused MD5 generation logic for InsecureKey

* Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/336421
* Follow-up for:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66371

**Problem**

FIPS environments don't support MD5 generation keys. Moreover, this
code is not used anymore.

**Solution**

Remove unused MD5 generation logic

Changelog: removed

lib/gitlab/insecure_key_fingerprint.rb

@@ -11,19 +11,12 @@ module Gitlab
   class InsecureKeyFingerprint
     attr_accessor :key
 
-    alias_attribute :fingerprint_md5, :fingerprint
-
-    #
     # Gets the base64 encoded string representing a rsa or dsa key
     #
     def initialize(key_base64)
       @key = key_base64
     end
 
-    def fingerprint
-      OpenSSL::Digest::MD5.hexdigest(Base64.decode64(@key)).scan(/../).join(':')
-    end
-
     def fingerprint_sha256
       Digest::SHA256.base64digest(Base64.decode64(@key)).scan(/../).join('').delete("=")
     end

spec/lib/gitlab/insecure_key_fingerprint_spec.rb

@@ -10,16 +10,9 @@ RSpec.describe Gitlab::InsecureKeyFingerprint do
     'Jw0='
   end
 
-  let(:fingerprint) { "3f:a2:ee:de:b5:de:53:c3:aa:2f:9c:45:24:4c:47:7b" }
   let(:fingerprint_sha256) { "MQHWhS9nhzUezUdD42ytxubZoBKrZLbyBZzxCkmnxXc" }
 
-  describe "#fingerprint" do
-    it "generates the key's fingerprint" do
-      expect(described_class.new(key.split[1]).fingerprint_md5).to eq(fingerprint)
-    end
-  end
-
-  describe "#fingerprint" do
+  describe '#fingerprint_sha256' do
     it "generates the key's fingerprint" do
       expect(described_class.new(key.split[1]).fingerprint_sha256).to eq(fingerprint_sha256)
     end