Remove the unnecessary completion item data field: decl_signature, signature, name

[jdneo]

See: https://github.com/eclipse/eclipse.jdt.ls/blob/c98fd2cb5ff7f442a2a17c2dbc382f238c1a0a7c/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/CompletionResolveHandler.java#L91-L93

https://github.com/eclipse/eclipse.jdt.ls/blob/c98fd2cb5ff7f442a2a17c2dbc382f238c1a0a7c/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/contentassist/CompletionProposalDescriptionProvider.java#L702-L724

The content can be simply calculated from the proposal cached in the CompletionResponse. As long as we can recover the completion proposal, there is no need to set them into the data field.