WL#5369: Remove --optimizer_search_depth=63

Remove the specific value 63 for the variable optimizer_search_depth.
Worklog 5252 deprecated this functionality, and this WL is created to follow up 
deprecation with actual removal.

Remove the specific value 63 for optimizer_search_depth, which enables the 
old optimizer behavior. The related function find_best() will also be removed.

Related bug cases:

BUG#50052 reports a regression in 6.0, --optimizer_search_depth=63 crashes. We 
can close this as "will not fix" if we deprecate this behavior in 5.1 and 5.5, 
and remove it in 6.0 (presuming that 6.0 will be the next major release after 
5.5).

Related tests:

greedy_optimizer - checks query cost for old and greedy cost-based optimizer 
subselect3 - tests that old optimizer works for semijoin conversion of subquery
optimizer_search_depth_basic - Checks legal values for variable

Related worklogs:

WL#5252: Deprecate --optimizer_search_depth=63
WL#5265: Remove deprecated 5.5 features