Since https://github.com/kubernetes/kubernetes/pull/112648, we can
efficiently handle selectors from pre-existing `map[string]string`,
making the cache obsolete.
Benchmark:
```
name old time/op new time/op delta
GetPodServiceMemberships-48 189µs ± 1% 193µs ± 1% +2.10% (p=0.000 n=10+10)
name old alloc/op new alloc/op delta
GetPodServiceMemberships-48 59.0kB ± 0% 58.9kB ± 0% -0.09% (p=0.000 n=9+9)
name old allocs/op new allocs/op delta
GetPodServiceMemberships-48 1.02k ± 0% 1.02k ± 0% ~ (all equal)
```
d9f2cc0c95