removed DaysSince primitive

[kkleidal]

Pull Request Description

Fixes #605

According to #605 , DaysSince is made obsolete by TimeSince, since DaysSince is equivalent to TimeSince(unit="days"). This patch removes the obsolete DaysSince primitive

---

After creating the pull request: in order to pass the changelog_updated check you will need to update the "Future Release" section of docs/source/changelog.rst to include this pull request.

[CLAassistant]

All committers have signed the CLA.

[kkleidal]

I can rebase down to one commit if that is standard practice and desired by the maintainers. Also: thanks for the coffee @kmax12

[codecov]

Codecov Report

Merging #628 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
- Coverage   97.43%   97.43%   -0.01%     
==========================================
  Files         118      118              
  Lines        9543     9534       -9     
==========================================
- Hits         9298     9289       -9     
  Misses        245      245

Impacted Files	Coverage Δ
...retools/primitives/standard/transform_primitive.py	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c5f9e53...5cf16a0. Read the comment docs.

[CJStadler]

LGTM!

Thanks for the contribution @kkleidal. You can squash when you merge.

[kkleidal]

Thanks @CJStadler . To which branch do you mean I should merge?

[CJStadler]

Sorry @kkleidal, I didn't realize you couldn't merge to master. Thanks again!