fix(kube_pod_tolerations): deduplicate tolerations before creating metric

[RiRa12621]

What this PR does / why we need it:
duplicate metrics currently create warnings in the logs of prometheus. Tolerations can be duplicated and kube-api doesn't want to change that behavior, hence we have to deduplicate here

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)
does not change cardinality

Which issue(s) this PR fixes
Fixes #2390

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[RiRa12621]

/assign @dgrisonnet

[mrueg]

The solution looks good to me.
Any thoughts on using slices.Sort + slices.Compact similar to #2502 ? This might be simpler than building a separate function for it.