Fix tests for NegativeLengthCheck.

Author: Max Schaefer

ql/test/query-tests/RedundantCode/NegativeLengthCheck/NegativeLengthCheck.expected

@@ -4,4 +4,3 @@
 | main.go:18:5:18:22 | ...<=... | 'len' is always non-negative, and hence cannot be less than -1. |
 | main.go:22:5:22:22 | ...==... | 'len' is always non-negative, and hence cannot equal -1. |
 | main.go:28:9:28:13 | ...<... | This unsigned value is always non-negative, and hence cannot be less than 0. |
-| main.go:36:9:36:15 | ...==... | This unsigned value is always non-negative, and hence cannot equal -1. |

ql/test/query-tests/RedundantCode/NegativeLengthCheck/main.go

@@ -31,7 +31,3 @@ func checkNegative(x uint) bool {
 func checkNonPositive(x uint) bool {
 	return x <= 0 // OK
 }
-
-func checkIsMinusOne(x uint) bool {
-	return x == -1 // NOT OK
-}