time: remove some unnecessary/duplicated global slices
time: remove some unnecessary/duplicated global slices

Removes two variables:

- days which is unused, and similar usage provided by longDayNames
- months in favour of using longMonthNames

Fixes #36359

Change-Id: I51b6b7408db9359c658462ba73e59ed432f655a6
GitHub-Last-Rev: 778d3ea157d363fcb5bced6d318381b44a1cac50
GitHub-Pull-Request: golang/go#36372
Reviewed-on: https://go-review.googlesource.com/c/go/+/213177
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Run-TryBot: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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