Mail 2.6.1 silences excessive warnings; remove Gemfile hack

Completes https://github.com/rails/rails/pull/15493

Revert "For our build, stick with mail 2.5.x for now"

This reverts commit b8f586a094c104006d29a87fee0d8b48d0af2d14.
Merge pull request #15573 from bf4/mail_2_6_1_released

Mail 2.6.1 silences excessive warnings; remove Gemfile hack