chore: remove ignore file caching entirely (#10813)

Effectively nobody uses it, it cost more than it was worth, the code
doesn't carry its weight. Remove it and let any similar mechanism in the
future be an internal implementation detail.

Closes #10812

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/ignore/cache.go

@@ -1,66 +0,0 @@
-// Copyright (C) 2014 The Syncthing Authors.
-//
-// This Source Code Form is subject to the terms of the Mozilla Public
-// License, v. 2.0. If a copy of the MPL was not distributed with this file,
-// You can obtain one at https://mozilla.org/MPL/2.0/.
-
-package ignore
-
-import (
-	"time"
-
-	"github.com/syncthing/syncthing/lib/ignore/ignoreresult"
-)
-
-type nower interface {
-	Now() time.Time
-}
-
-var clock = nower(defaultClock{})
-
-type cache struct {
-	entries map[string]cacheEntry
-}
-
-type cacheEntry struct {
-	result ignoreresult.R
-	access int64 // Unix nanosecond count. Sufficient until the year 2262.
-}
-
-func newCache() *cache {
-	return &cache{
-		entries: make(map[string]cacheEntry),
-	}
-}
-
-func (c *cache) clean(d time.Duration) {
-	for k, v := range c.entries {
-		if clock.Now().Sub(time.Unix(0, v.access)) > d {
-			delete(c.entries, k)
-		}
-	}
-}
-
-func (c *cache) get(key string) (ignoreresult.R, bool) {
-	entry, ok := c.entries[key]
-	if ok {
-		entry.access = clock.Now().UnixNano()
-		c.entries[key] = entry
-	}
-	return entry.result, ok
-}
-
-func (c *cache) set(key string, result ignoreresult.R) {
-	c.entries[key] = cacheEntry{result, time.Now().UnixNano()}
-}
-
-func (c *cache) len() int {
-	l := len(c.entries)
-	return l
-}
-
-type defaultClock struct{}
-
-func (defaultClock) Now() time.Time {
-	return time.Now()
-}