Cleanup YGValue.h header (#37179)

Summary:
X-link: https://github.com/facebook/react-native/pull/37179

Pull Request resolved: https://github.com/facebook/yoga/pull/1273

1. Simplify nan handling a bit
2. Remove string literal operators which seem dead enough to remove. These are public, so anyone could be using them, but it seems like almost nobody is.
    1. FB has no usages of `using namespace facebook::yoga::literals` (exposing the literal operators) outside of Yoga tests.
    1. There is only [a single usage](6dfba905ea/SDLTest/UIKit.hpp (L19)) on GitHub.

Reviewed By: yungsters

Differential Revision: D45419970

fbshipit-source-id: 8121303e5ae66596132a848a711802081728f4fb

tests/YGValueTest.cpp

@@ -21,29 +21,3 @@ TEST(YGValue, supports_equality) {
       (YGValue{YGUndefined, YGUnitUndefined}));
   ASSERT_EQ((YGValue{0, YGUnitAuto}), (YGValue{-1, YGUnitAuto}));
 }
-
-using namespace facebook::yoga::literals;
-
-TEST(YGValue, supports_double_point_literals) {
-  ASSERT_EQ(12.5_pt, (YGValue{12.5, YGUnitPoint}));
-}
-
-TEST(YGValue, supports_double_percent_literals) {
-  ASSERT_EQ(12.5_percent, (YGValue{12.5, YGUnitPercent}));
-}
-
-TEST(YGValue, supports_integral_point_literals) {
-  ASSERT_EQ(34_pt, (YGValue{34, YGUnitPoint}));
-}
-
-TEST(YGValue, supports_integral_percent_literals) {
-  ASSERT_EQ(125_percent, (YGValue{125, YGUnitPercent}));
-}
-
-TEST(YGValue, supports_negative_point_literals) {
-  ASSERT_EQ(-34.5_pt, (YGValue{-34.5, YGUnitPoint}));
-}
-
-TEST(YGValue, supports_negative_percent_literals) {
-  ASSERT_EQ(-34.5_percent, (YGValue{-34.5, YGUnitPercent}));
-}