logging: quiet 'removing n old cache dirs'

Closes #3595

Choosing to include `stdoutIsTerminal()` as:
 - all other instances with `!opts.JSON` do so
 - this likely will not affect anything, especially when autorun
 - this seems to not be a meaningful enough summary
     to include in auto-backup reports

JSON is still likely not guaranteed to work and this is a suboptimal
  solution to this. Ideally, #1804 should refactor all print statements,
  and define+document(+handle) when stdoutIsTerminal() should be used.
  Else, it may end up more inconsistent and bulky
  (duplicate lines, longer files).

cmd/restic/global.go

@@ -502,8 +502,9 @@ func OpenRepository(opts GlobalOptions) (*repository.Repository, error) {
 
 	// cleanup old cache dirs if instructed to do so
 	if opts.CleanupCache {
-		Printf("removing %d old cache dirs from %v\n", len(oldCacheDirs), c.Base)
-
+		if stdoutIsTerminal() && !opts.JSON {
+			Verbosef("removing %d old cache dirs from %v\n", len(oldCacheDirs), c.Base)
+		}
 		for _, item := range oldCacheDirs {
 			dir := filepath.Join(c.Base, item.Name())
 			err = fs.RemoveAll(dir)