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_test.go

@@ -1,94 +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 (
-	"testing"
-	"time"
-
-	"github.com/syncthing/syncthing/lib/ignore/ignoreresult"
-)
-
-func TestCache(t *testing.T) {
-	fc := new(fakeClock)
-	oldClock := clock
-	clock = fc
-	defer func() {
-		clock = oldClock
-	}()
-
-	c := newCache()
-
-	res, ok := c.get("nonexistent")
-	if res.IsIgnored() || res.IsDeletable() || ok {
-		t.Errorf("res %v, ok %v for nonexistent item", res, ok)
-	}
-
-	// Set and check some items
-
-	c.set("true", ignoreresult.IgnoredDeletable)
-	c.set("false", 0)
-
-	res, ok = c.get("true")
-	if !res.IsIgnored() || !res.IsDeletable() || !ok {
-		t.Errorf("res %v, ok %v for true item", res, ok)
-	}
-
-	res, ok = c.get("false")
-	if res.IsIgnored() || res.IsDeletable() || !ok {
-		t.Errorf("res %v, ok %v for false item", res, ok)
-	}
-
-	// Don't clean anything
-
-	c.clean(time.Second)
-
-	// Same values should exist
-
-	res, ok = c.get("true")
-	if !res.IsIgnored() || !res.IsDeletable() || !ok {
-		t.Errorf("res %v, ok %v for true item", res, ok)
-	}
-
-	res, ok = c.get("false")
-	if res.IsIgnored() || res.IsDeletable() || !ok {
-		t.Errorf("res %v, ok %v for false item", res, ok)
-	}
-
-	// Sleep and access, to get some data for clean
-
-	*fc += 500 // milliseconds
-
-	c.get("true")
-
-	*fc += 100 // milliseconds
-
-	// "false" was accessed ~600 ms ago, "true" was accessed ~100 ms ago.
-	// This should clean out "false" but not "true"
-
-	c.clean(300 * time.Millisecond)
-
-	// Same values should exist
-
-	_, ok = c.get("true")
-	if !ok {
-		t.Error("item should still exist")
-	}
-
-	_, ok = c.get("false")
-	if ok {
-		t.Errorf("item should have been cleaned")
-	}
-}
-
-type fakeClock int64 // milliseconds
-
-func (f *fakeClock) Now() time.Time {
-	t := time.Unix(int64(*f)/1000, (int64(*f)%1000)*int64(time.Millisecond))
-	*f++
-	return t
-}