Remove tests for ignore_missing_libraries

The deprecation is causing these tests to fail.

Library/Homebrew/test/formula_spec.rb

@@ -1841,54 +1841,6 @@ describe Formula do
     end
   end
 
-  describe "#ignore_missing_libraries" do
-    after do
-      Homebrew::SimulateSystem.clear
-    end
-
-    it "adds library to allowed_missing_libraries on Linux", :needs_linux do
-      Homebrew::SimulateSystem.clear
-      f = formula do
-        url "foo-1.0"
-
-        ignore_missing_libraries "bar.so"
-      end
-      expect(f.class.allowed_missing_libraries.to_a).to eq(["bar.so"])
-    end
-
-    it "adds library to allowed_missing_libraries on macOS when simulating Linux", :needs_macos do
-      Homebrew::SimulateSystem.os = :linux
-      f = formula do
-        url "foo-1.0"
-
-        ignore_missing_libraries "bar.so"
-      end
-      expect(f.class.allowed_missing_libraries.to_a).to eq(["bar.so"])
-    end
-
-    it "raises an error on macOS", :needs_macos do
-      Homebrew::SimulateSystem.clear
-      expect {
-        formula do
-          url "foo-1.0"
-
-          ignore_missing_libraries "bar.so"
-        end
-      }.to raise_error("ignore_missing_libraries is available on Linux only")
-    end
-
-    it "raises an error on Linux when simulating macOS", :needs_linux do
-      Homebrew::SimulateSystem.os = :macos
-      expect {
-        formula do
-          url "foo-1.0"
-
-          ignore_missing_libraries "bar.so"
-        end
-      }.to raise_error("ignore_missing_libraries is available on Linux only")
-    end
-  end
-
   describe "#generate_completions_from_executable" do
     let(:f) do
       Class.new(Testball) do