Bug#916832: gcc-6: keep out of testing

[Emilio Pozuelo Monfort <pochu@debian.org>]

Source: gcc-6
Severity: serious

Nothing is using GCC 6 anymore, so we are removing it from testing. We
will ship buster with GCC 8 and most likely GCC 7. This bug is to prevent
GCC 6 from migrating back to testing.

Emilio

-- System Information:
Debian Release: buster/sid
  APT prefers unstable
  APT policy: (800, 'unstable'), (700, 'experimental'), (650, 'testing'), (500, 'unstable-debug'), (500, 'testing-debug')
Architecture: amd64 (x86_64)
Foreign Architectures: i386, armhf

Kernel: Linux 4.18.0-2-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.utf8 (charmap=UTF-8), LANGUAGE=en_GB.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled