Further simplify regex

Reduces fix time by about 30% in the test suite.

```
goos: darwin
goarch: amd64
pkg: github.com/automattic/go-search-replace
BenchmarkFix-4                      1000000          2074 ns/op         216 B/op           7 allocs/op
BenchmarkSimpleReplace-4            2000000           605 ns/op         480 B/op           7 allocs/op
BenchmarkSerializedReplace-4         500000          3589 ns/op         696 B/op          14 allocs/op
PASS
ok      github.com/automattic/go-search-replace    5.786s
```

```
goos: darwin
goarch: amd64
pkg: github.com/automattic/go-search-replace
BenchmarkFix-4                      1000000          1484 ns/op          88 B/op           4 allocs/op
BenchmarkSimpleReplace-4            2000000           624 ns/op         480 B/op           7 allocs/op
BenchmarkSerializedReplace-4         500000          3085 ns/op         568 B/op          11 allocs/op
PASS
ok      github.com/automattic/go-search-replace    4.976s
```

search-replace.go

@@ -12,7 +12,7 @@ import (
 
 const (
 	searchRe  = `s:\d+:\\\".*?\\\";`
-	replaceRe = `(?:s:)(?:\d+:)(\\\")(.*?)(\\\";)`
+	replaceRe = `s:\d+:\\\"(.*?)\\\";`
 
 	badInputRe   = `\w:\d+:`
 	inputRe      = `^[A-Za-z0-9_\-\.:/]+$`
@@ -128,14 +128,14 @@ func replaceAndFix(line string, replacements map[string]string) string {
 func fix(match string) string {
 	parts := replace.FindStringSubmatch(match)
 
-	if len(parts) < 3 {
+	if len(parts) != 2 {
 		// This looks wrong, don't touch anything
 		return match
 	}
 
 	// Get string length - number of escaped characters and avoid double counting escaped \
-	length := len(parts[2]) - (strings.Count(parts[2], `\`) - strings.Count(parts[2], `\\`))
-	return fmt.Sprintf("s:%d:%s%s%s", length, parts[1], parts[2], parts[3])
+	length := len(parts[1]) - (strings.Count(parts[1], `\`) - strings.Count(parts[1], `\\`))
+	return fmt.Sprintf("s:%d:\\\"%s\\\";", length, parts[1])
 }
 
 func validInput(in string, length int) bool {