default-distrovars: Clean up gcc license issues

default-distrovars: Clean up gcc license issues

There are several entries here which are not needed with the
modern license handling code:

gcc-source - moved to direct handling in base.bbclass
             (due to version appended to the name)
libgcc - Listed as GPLv3 exception for its packages
libgcc-initial - Listed as GPLv3 exception
gcc-runtime - Indivisual packages listed as GPLv3 exception where appropriate

(From OE-Core rev: 48c4922ab921a1bb2103cc331d0839febd36beb8)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>