delete buggy stddev function from MovingAverage #293

Job	Run time
Apache2 tests on Linux	2m 18s
Apache2 tests on macOS	3m 12s
C++ tests on Linux	2m 52s
C++ tests on macOS	4m 54s
Homebrew packaging tests	6m 24s
Nginx tests on Linux	5m 35s
Nginx dynamic module tests on Linux	1m 18s
Nginx tests on macOS	2m 46s
Node.js tests on Linux	25s
Nginx dynamic module tests on macOS	1m 7s
Node.js tests on macOS	23s
Ruby tests on Linux	1m 18s
Source packaging tests	27s
Passenger Standalone tests on Linux	4m 34s
Ruby tests on macOS	1m 21s
Passenger Standalone tests on macOS	4m 34s
	43m 28s

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