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 <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>