Revert "TravisCI: Explicitly install libc6-dbg for Valgrind."
Revert "TravisCI: Explicitly install libc6-dbg for Valgrind."

libc6-dbg gets pulled by dependency from Valgrind, but is not,
because there is some inconsistency in the package repository.

This reverts commit 3d34ddb84bd73047896a54a6a0d3a64c9f5bd2ef.

besser82/libxcrypt,besser82/libxcrypt,besser82/libxcrypt,besser82/libxcrypt