Radundant objects of `FetchSourceContext` class

[urmichm]

Is your feature request related to a problem? Please describe

Fallow up of #20612
Usage of boolean constructor is redundant

OpenSearch/server/src/main/java/org/opensearch/search/fetch/subphase/FetchSourceContext.java

Line 80 in 59be6ae

public FetchSourceContext(boolean fetchSource) {

Since there are already the immutable objects:

OpenSearch/server/src/main/java/org/opensearch/search/fetch/subphase/FetchSourceContext.java

Line 67 in 59be6ae

public static final FetchSourceContext FETCH_SOURCE = new FetchSourceContext(true);

OpenSearch/server/src/main/java/org/opensearch/search/fetch/subphase/FetchSourceContext.java

Line 68 in 59be6ae

public static final FetchSourceContext DO_NOT_FETCH_SOURCE = new FetchSourceContext(false);

Extra (unnecessary) objects inflate the memory.

Describe the solution you'd like

Make the boolean constructor private and use FETCH_SOURCE and DO_NOT_FETCH_SOURCE

Related component

Other

Describe alternatives you've considered

No response

Additional context

No response