Clean up SQL generation in MySQL platform

[morozov]

Q	A
Type	improvement
BC Break	no

As identified in #4978 (comment), the query generated by getListTableForeignKeysSQL() uses a DISTINCT clause which requires selecting the columns used in the ORDER BY clause.

What has been changed:

1. The DISTINCT clause has been removed since it's not necessary.
2. The WHERE k.REFERENCED_COLUMN_NAME IS NOT NULL condition has been removed since it's not necessary.
3. The WHERE c.constraint_schema = $databaseNameSql condition has been replaced with an analogous ON condition.
4. The original (upper) column case is used with the redundant quotes removed.

Additionally, the following has been done in multiple queries:

1. The logic of quoting the database name or defaulting to DATABASE() has been extracted into a private method.
2. All occurrences like $string = quote($string) have been replaced with inlining with the concatenation to avoid potential double quoting.

The last two changes likely would have happened in #4882 but it's better to do the cleanup separately.