Bug 7188: C4::Context::db_scheme2dbi does case-insensitive grep

Bug 7188: C4::Context::db_scheme2dbi does case-insensitive grep

Checking NYTProf in C4::Context, it appears that the /i flag is highly time-consumming
As we don't support anything outside from mysql, returning directly mysql directly

If in a future version PostgresSQL works, then we will have to update this sub. But if an ORM is introduced, this code will be removed completly anyway

Testing note:
Drops execution time of the line from 300ms to 20 microseconds (in my testing).