Revert "mysql2 adapter fixed upstream to delegate quoting of BigDecimal." Breaks booleans in PostgreSQL

This reverts commit b4be619650bd8c7b85735eb290bc0257094e3da0.

Conflicts:

	activerecord/test/cases/relation/where_test.rb
Revert "active_record: Quote numeric values compared to string columns." This
breaks booleans in PostgreSQL

This reverts commit 0fc58caf184c77cd741e92655f33e34f554b025b.