WL#10824: Remove query cache

We've decided to remove the query cache for 8.0 (it will be deprecated in 5.7, 
which this worklog does not concern itself with).

The proposed design:

 * HAVE_QUERY_CACHE will now return NO, so that well-behaved clients can check 
for this and behave accordingly.
 * The SQL_NO_CACHE keyword will continue to exist, but will be ignored (no 
effect in the grammar). This is so that e.g. mysqldump can continue working. 
(SQL_CACHE will be removed as a token.)
 * The RESET QUERY CACHE and FLUSH QUERY CACHE statements will be removed. Trying 
to use them will return errors.
 * All query cache command-line options will be removed. Trying to set them will 
return errors.
 * All query cache system variables will be removed. Trying to set them will 
return errors.
 * All tests that are for testing query cache will be removed. Some tests use 
the query cache as a means to other ends (e.g. reducing I/O); we will need to 
look at these on a case-by-case basis.
 * All RQG grammars that set SQL_CACHE/SQL_NO_CACHE will have these parts 
removed.

Apart from that, the procedure is quite obvious; we just remove all query cache-
related code.