gce: rm unnecessary GetMigInstanceTemplate locking

The lock held from (and covering most of) `GetMigInstanceTemplate()` prevents
`InstanceTemplates.Get()` concurrent executions, while it's not really needed
as all caches access functions used here already take care of locking cache
accesses (and it's reading and filling one single cache entry per call).

`InstanceTemplates.Get()` isn't fast (at about 0.5s/call in my test env),
running all calls sequentially slows down startup significantly:

```
forceRefresh()
  fetchAutoMigs()
    workqueue.ParallelizeUntil(..., func() { <- can launch concurrent registerMigs
      registerMig()
        GetMigTemplateNode()
          GetMigInstanceTemplate() <- lock held
            FetchMigTemplate()
              client.gceService.InstanceTemplates.Get() <- slow API call
```

When running this change on a cluster having 800 MIGs, startup went from 6min to 1.5min.
Also tested with a `-race` build.

cluster-autoscaler/cloudprovider/gce/mig_info_provider.go

@@ -244,9 +244,6 @@ func (c *cachingMigInfoProvider) GetMigInstanceTemplate(migRef GceRef) (*gce.Ins
 		return nil, err
 	}
 
-	c.migInfoMutex.Lock()
-	defer c.migInfoMutex.Unlock()
-
 	template, found := c.cache.GetMigInstanceTemplate(migRef)
 	if found && template.Name == templateName {
 		return template, nil