Remove deadcode (#22245)

- Remove code that isn't being used.

Found this is my stash from a few weeks ago, not sure how I found this
in the first place.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/markup/markdown/markdown_test.go

@@ -74,28 +74,6 @@ func TestRender_StandardLinks(t *testing.T) {
 		`<p><a href="`+lnkWiki+`" rel="nofollow">WikiPage</a></p>`)
 }
 
-func TestMisc_IsMarkdownFile(t *testing.T) {
-	setting.Markdown.FileExtensions = []string{".md", ".markdown", ".mdown", ".mkd"}
-	trueTestCases := []string{
-		"test.md",
-		"wow.MARKDOWN",
-		"LOL.mDoWn",
-	}
-	falseTestCases := []string{
-		"test",
-		"abcdefg",
-		"abcdefghijklmnopqrstuvwxyz",
-		"test.md.test",
-	}
-
-	for _, testCase := range trueTestCases {
-		assert.True(t, IsMarkdownFile(testCase))
-	}
-	for _, testCase := range falseTestCases {
-		assert.False(t, IsMarkdownFile(testCase))
-	}
-}
-
 func TestRender_Images(t *testing.T) {
 	setting.AppURL = AppURL
 	setting.AppSubURL = AppSubURL