Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination

Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination

fc069a3a6319 implements Self-Join Elimination (SJE) and provides a new GUC
variable: enable_self_join_elimination.  This new GUC variable was marked
as GUC_NOT_IN_SAMPLE.  However, enable_self_join_elimination is documented
and is not different from any other enable_* GUCs.  Thus, remove
GUC_NOT_IN_SAMPLE from it and add it to the postgresql.conf.sample.

Discussion: https://postgr.es/m/CAPpHfdsqMTEsmxk3aQwt6xPz%2BKpUELO%3D6fzmER9ZRGrbs4uMfA%40mail.gmail.com
Author: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>