doc/go1.15: remove TODO intended for the Core library section
The TODO was added durring the initial creation of the document.
In the current location, it makes it seem like the tzdata documents
are incomplete when they are complete. It is understood that the
entire Core library section will be a work in progress until the release.
For #37419
Change-Id: Ic857eb0ec2583781c701985ea62e519e9d940090
Reviewed-on: https://go-review.googlesource.com/c/go/+/236760 Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>