There was an incomplete reset of the name resolution context, that caused 
INSERT ... SELECT ... JOIN statements to resolve not by joint row type calculated
for the join.
Removed the redundant re-initialization of the context, because 
mysql_insert_select_prepare() now correctly saves/restores the context.


mysql-test/r/insert_select.result:
  Bug #18080: INSERT ... SELECT ... JOIN results in ambiguous field list error
  
  - testsuite for the bug
mysql-test/t/insert_select.test:
  Bug #18080: INSERT ... SELECT ... JOIN results in ambiguous field list error
  
  - testsuite for the bug
sql/sql_parse.cc:
  Bug #18080: INSERT ... SELECT ... JOIN results in ambiguous field list error
  
  - remove an incomplete reset of the context because the mentioned function
  correctly saves/restores the context.