sdktool: Fix addKit complaining about unknown toolchains

Fixes a regression in 07f7dd9c1434738ad59467ac8830be41db03dfa0
that caused the Qt SDK installer to complain e.g. with

  Toolchain x86-linux-generic-elf-86bit does not exist.

Qt Creator has code to automatically detect toolchains, so
it's not necessary for every toolchain to be explicitly registered.

Task-number: QTBUG-37875
Change-Id: I1e8909da2ff919a8cccdf86bb3a2f7a5cc79a257
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>