Include mysqld_error.h has been here for ages but was superfluous

[sodabrew]

Following up on #895

[sodabrew]

Verified that no constants from this file are in use:

awk '/define/ { print $2 }' /opt/local/include//mysql56/mysql/mysqld_error.h | xargs -I{} -- ag -ai {} ext/ lib/

Tested across several versions of MySQL.

[jaredbeck]

Testing 763235e against MariaDB 10.2.10 installed by Homebrew, looks good 👍

git clone https://github.com/brianmario/mysql2.git
...
git show
commit 763235ec8311a28c8959628fee1fec4f07edfafa
...
gem build mysql2.gemspec 
...
gem install mysql2-0.4.9.gem 
Building native extensions. This could take a while...
Successfully installed mysql2-0.4.9
1 gem installed
mysql -v
Server version: 10.2.10-MariaDB Homebrew