Remove "SQL" prefix from "SQLDateTimeField" struct

[benesch]

I realized a moment too late that I'd missed a type name in
when removing the "SQL" prefix from types in ac555d7. As far as I can
tell, this was the only oversight.

[coveralls]

Pull Request Test Coverage Report for Build 380

• 27 of 27 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.239%

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Totals
Change from base Build 378:	0.0%
Covered Lines:	4326
Relevant Lines:	4690

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💛 - Coveralls

[nickolay]

LGTM! (Not sure there’s a point in reviewing changes like this before merging.)

[benesch]

(Not sure there’s a point in reviewing changes like this before merging.)

Sure, happy to just merge tiny stuff like this without waiting for approval!