runtime(sql, mysql): fix E169: Command too recursive with sql_type_default = "mysql"

runtime(sql, mysql): fix E169: Command too recursive with sql_type_default = "mysql"

Problem: When setting "let g:sql_type_default = "mysql", editing .sql
file reports "E169: Command too recursive" error

Solution:

- Add 'let b:did_ftplugin = 1' at the top of ftplugin/sql.vim
- Add 'if exists("b:did_ftplugin") | finish | endif' in ftplugin/mysql.vim
- Add missing header information in ftplugin/mysql.vim
- Remove redundant code in ftplugin/sql.vim

fixes: #15474
closes: #15475

Signed-off-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>
Signed-off-by: Riley Bruins <ribru17@hotmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>