Avoid explicitly inject ObjectMapper to the ObjectMapper

[sammyhk]

ConfiguringObjectMapperProvider explicitly inject ObjectMapper to the ObjectMapper:
https://github.com/graphql-java-kickstart/graphql-java-servlet/blob/master/src/main/java/graphql/servlet/ConfiguringObjectMapperProvider.java#L27
Which is used in VariablesDeserializer:
https://github.com/graphql-java-kickstart/graphql-java-servlet/blob/master/src/main/java/graphql/servlet/internal/VariablesDeserializer.java#L18

Checked Jackson 2 API, this is not needed by using ObjectCodec API (abstract class of ObjectMapper) which can achieve the same goal.
Example (Before):

objectMapper.readValue(str, typeReference);

(After):

objectCodec.readValue(objectCodec.getFactory().createParser(str), typeReference);