Remove duplicated parameter check on #where!

It seems that #where! is not designed to be used as a chained where.
See initial implementation at 8c2c60511beaad05a218e73c4918ab89fb1804f0.
So, no need to check twice.

We should not test #where!

https://github.com/rails/rails/pull/15285#discussion_r13018316
Merge pull request #15285 from sergey-alekseev/remove-active-record-where-duplicated-condition

Remove duplicated parameter check on #where!