Remove outdated reference to a removed file

parse_jsontable.c was removed as part of 2f2b18bd3f, though its
mention in src/backend/parser/README was not.  Fix that.

Discussion: https://postgr.es/m/CA%2BHiwqHDzw8AP8p_dEkFr0xg458ZTf58zbivAHhK4UeNrx9Tdg%40mail.gmail.com

src/backend/parser/README

@@ -20,7 +20,6 @@ parse_cte.c	handle Common Table Expressions (WITH clauses)
 parse_expr.c	handle expressions like col, col + 3, x = 3 or x = 4
 parse_enr.c	handle ephemeral named rels (trigger transition tables, ...)
 parse_func.c	handle functions, table.column and column identifiers
-parse_jsontable.c handle JSON_TABLE
 parse_merge.c	handle MERGE
 parse_node.c	create nodes for various structures
 parse_oper.c	handle operators in expressions