feat: Remove task_pruning_cache_fetch_batch_size in favor of existing memcached batching

[salvacorts]

What this PR does / why we need it:

Removing the new config introduced in #21548

Since we already have the memcached batchsize param. Which was actually preventing anny efective batching though task_pruning_cache_fetch_batch_size.

loki/pkg/storage/chunk/cache/memcached.go

Line 25 in 8509f1d

BatchSize int `yaml:"batch_size"`

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

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Note

Medium Risk
Removes configurable batching for cache lookups during plan-time empty-task pruning and switches to a single Fetch call per cache name, which could change cache load/performance characteristics for large plans depending on backend behavior.

Overview
Removes the experimental task_pruning_cache_fetch_batch_size configuration/flag and its documentation.

Plan-time pruning of empty cached tasks now performs a single cache Fetch per cache name (no explicit key batching), drops the associated batching plumbing in workflow options/planner, and deletes the batching-focused unit test.

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[github-actions]

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