1) No mutex and no function call if we're not using
plugins.
2) If we're above the table definition cache limit,
delete the oldest unused share, not the share on our hands.

sql/sql_base.cc:
  If we're above the table definition cache limit,
  delete the oldest unused share, not the share on our hands.
sql/sql_lex.cc:
  No mutex and no function call if we're not using
  plugins.