Revert "active_record: Quote numeric values compared to string fields for mysql."

This reverts commit d0223ebffc154b221ce8bfba3d2d0d24f760c6f1.
active_record: Delegate BigDecimal quoting to abstract adapter.

The abstract connection adapter already supports BigDecimal, and removing
this code from the mysql2 adapter allows proper quoting when comparing with
a string column.