WL#8396: Deprecate and remove temp-pool

When investigating changing the default setting for temp-pool, it seems clear
that this setting was introduced to compensate for a Linux discrepancy.

Documentation for --temp-pool:
http://dev.mysql.com/doc/refman/5.7/en/server-options.html#option_mysqld_temp-pool

It was introduced in MySQL 3.23 with the following comment:

"Added --temp-pool option to mysqld. Using this option causes most
temporary files created to use a small set of names, rather than a
unique name for each new file. This is to work around a problem in
the Linux kernel dealing with creating a bunch of new files with
different names. With the old behavior, Linux seems to "leak" memory,
as it's being allocated to the directory entry cache instead of the
disk cache."

In MySQL 4.0.3 (Aug 2002) it was set as default for Linux.

In all supported Linuxes, the issue at hand is fixed, so there is no need for
this setting.

temp-pool is deprecated in 5.7 and removed in 8.0

User Documentation
==================

http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-18.html
http://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-1.html

NF-1: No change in performance. Only relevant on Linux.