remove unnecessary usages of NodeFields and NodeChildren

...acle/truffle/r/nodes/builtin/base/BaseGammaFunctions.java

...cle/truffle/r/nodes/builtin/base/infix/SpecialsUtils.java

.../oracle/truffle/r/nodes/builtin/base/infix/Subscript.java

...e/truffle/r/nodes/builtin/base/infix/UpdateSubscript.java

...des/src/com/oracle/truffle/r/nodes/binary/BinaryNode.java

...rc/com/oracle/truffle/r/nodes/unary/CastToVectorNode.java

.../com/oracle/truffle/r/nodes/unary/ConvertBooleanNode.java

...es/src/com/oracle/truffle/r/nodes/unary/FirstIntNode.java

...src/com/oracle/truffle/r/nodes/unary/FirstStringNode.java