Revert "Minor internal/reader/readability/readability.go speedup"

This reverts commit 4db138d4b87c988eed6dbe2fc72cf1a13d393d8b.

```
panic: runtime error: index out of range [-1]

goroutine 49 [running]:
miniflux.app/v2/internal/reader/readability.getArticle.func1(0x8?, 0xc000b56570)
        /home/fred/repos/miniflux/v2/internal/reader/readability/readability.go:120 +0x2ac
github.com/PuerkitoBio/goquery.(*Selection).Each(0xc000b56510, 0xc000892fa8)
        /home/fred/go/pkg/mod/github.com/!puerkito!bio/goquery@v1.9.0/iteration.go:10 +0x62
miniflux.app/v2/internal/reader/readability.getArticle(0xc00044f1f0, 0xc000a04a50)
        /home/fred/repos/miniflux/v2/internal/reader/readability/readability.go:101 +0x15d
miniflux.app/v2/internal/reader/readability.ExtractContent({0x1005d00?, 0xc0001522d0?})
        /home/fred/repos/miniflux/v2/internal/reader/readability/readability.go:91 +0x211
miniflux.app/v2/internal/reader/scraper.ScrapeWebsite(0xc000893688?, {0xc0007ce720, 0x54}, {0x0, 0x0})
        /home/fred/repos/miniflux/v2/internal/reader/scraper/scraper.go:63 +0x859
miniflux.app/v2/internal/reader/processor.ProcessFeedEntries(0xc000133188, 0xc000502c40, 0xc0003e6360, 0x0)
        /home/fred/repos/miniflux/v2/internal/reader/processor/processor.go:77 +0x8ea
miniflux.app/v2/internal/reader/handler.RefreshFeed(0xc000133188, 0x10cf, 0x52d5c, 0x0)
        /home/fred/repos/miniflux/v2/internal/reader/handler/handler.go:301 +0x1485
miniflux.app/v2/internal/cli.refreshFeeds.func1(0x0)
        /home/fred/repos/miniflux/v2/internal/cli/refresh_feeds.go:59 +0x2d7
created by miniflux.app/v2/internal/cli.refreshFeeds in goroutine 1
        /home/fred/repos/miniflux/v2/internal/cli/refresh_feeds.go:50 +0x5d5
```

internal/reader/readability/readability.go

@@ -21,7 +21,8 @@ const (
 )
 
 var (
-	divToPElementsRegexp = regexp.MustCompile(`(?i)<(?:a|blockquote|dl|div|img|ol|p|pre|table|ul)`)
+	divToPElementsRegexp = regexp.MustCompile(`(?i)<(a|blockquote|dl|div|img|ol|p|pre|table|ul)`)
+	sentenceRegexp       = regexp.MustCompile(`\.( |$)`)
 
 	blacklistCandidatesRegexp  = regexp.MustCompile(`(?i)popupbody|-ad|g-plus`)
 	okMaybeItsACandidateRegexp = regexp.MustCompile(`(?i)and|article|body|column|main|shadow`)
@@ -113,11 +114,9 @@ func getArticle(topCandidate *candidate, candidates candidateList) string {
 			content := s.Text()
 			contentLength := len(content)
 
-			if contentLength >= 80 {
-				if linkDensity < .25 {
-					append = true
-				}
-			} else if linkDensity == 0 && (content[len(content)-1] == '.' || strings.Contains(content, ". ")) {
+			if contentLength >= 80 && linkDensity < .25 {
+				append = true
+			} else if contentLength < 80 && linkDensity == 0 && sentenceRegexp.MatchString(content) {
 				append = true
 			}
 		}