Revert "Merge pull request #6197 from blakesmith/connection_adapters_without_explain_support"

This reverts commit 29d564add926307fea8cd3f77caf88d735e252a1.

This Rails upstream commit, added after 3.2.12 and included in 3.2.13, mistakenly opens a database connection to check for auto explain support.
This causes issues if you use threads as each will make a database connection for any queries, even cached ones.

It turns out that the upstream Rails 4-0-stable branch has this issue fixed as they removed the auto explain feature entirely: https://github.com/rails/rails/pull/9400
That pull references this issue, https://github.com/rails/rails/issues/9385, where a database connection was required during assets precompiling following the commit we are reverting.