toolchain-external: fix bfin external toolchain download

Fixes http://autobuild.buildroot.org/results/7d0/7d0eee796407880a4554474e5affbe98b8caa271/



Commit 1cbffbd0 (eliminate double slashes caused by FOO_SITE ending in
a slash) added a double replacement for the special bfin external
toolchain download, breaking it.

Fix it by removing the 2nd replacement.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>