We have recently done significant refactoring to ensure that the
objects TABLE_LIST::grant and TABLE::grant are kept synchronized for
a specific use of a table. This patch goes one step further by removing
the TABLE::grant object and relying solely on TABLE_LIST::grant.

The obvious benefits are less work, and less complexity to keep the
two objects synchronized.

Implementation is straightforward, except for the HANDLER interface
which uses two TABLE_LIST objects to manage a table. Solution here is
the simplest possible (keep the two GRANT objects synchronized), since
cleanup of HANDLER is not prioritized. Notice also that columns specified
in HANDLER READ statements cannot be privilege checked due to bug no.
25987758, however this is not critical as all columns are nevertheless
checked by insert_fields().

Pay attention to the big if statement in insert_fields() that
proved to be far too complex and pretty misleading.