toolchain-external: remove Sourcery PowerPC toolchains

toolchain-external: remove Sourcery PowerPC toolchains

These are running long on the teeth - the bundled (e)glibc versions
are very old with several security bugs, they don't work reliably with
-Os and have several build failures related to internal compiler
errors such as:

http://autobuild.buildroot.net/results/fe7/fe7bdba5faf199275aedea2918705b5d19d228bf/
http://autobuild.buildroot.net/results/935/935ac42c30ed893939c06c077534f060aed80e9a/
http://autobuild.buildroot.net/results/a47/a476af82c8fe4a279117314b278b08af9a08fe54/
http://autobuild.buildroot.net/results/cae/cae720b5096be2672b4dc1311ae3fc4ed06a3b53/

The situation will not provide, and will in fact get worse with older kernel
headers precluding modern package versions and the old gcc version doing as
well so remove them.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
[Thomas:
 - remove no longer needed comment in liquid-dsp Config.in file, as
   noticed by Romain Naour.
 - add Config.in.legacy options, as noticed by Romain Naour.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>