Don't use array_merge in getAllSnaks methods

It is unnecessary and more costly, performance-wise.
array_merge consumes the most memory of any method
used throughout the data model, Wikibase, and MediaWiki.

Avoiding array_merge here also helps improve the time it
takes to load an item in Wikibase.

In a medium-size item, with array_merge:

Entity::getAllSnaks - called 2 times, takes 109 ms
Claim::getAllSnaks - called 846 times, takes 188 ms
Statement::getAllSnaks - called 846 times, takes 148 ms

Without array_merge, on the same item:

Entity::getAllSnaks - 22 ms
Claim::getAllSnaks - 63 ms
Statement::getAllSnaks - 68 ms

StatementList::getAllSnaks appears unused