Consolidate getExecutorFind() and _getExecutorFind()

These two functions (one with the underscore and one without) used to be necessary in order to support the code path for legacy OP_QUERY find. In particular, both getExecutorFind() and getExecutorLegacyFind() used to call _getExecutorFind(). However, the code for legacy OP_QUERY find was deleted under SERVER-57391. This means that _getExecutorFind() has just one caller, so it would be clearer if we combine it with getExecutorFind().