This reverts the commits for the master branch of RBS:

- commit f717faac: "Update rbs
  revision to test"

    The target revision to test is in master branch, not for 3.2.x.

- commit 9e93af53: "Skip RBS
  `RbConfig::TOPDIR` test that is `nil` before installation"

    RbConfig_test.rb is not updated in 3.2.x branch.