slang: Clean up the additions to PREMIRRORS

Since commit 044fb04d in bitbake (fetch2: Allow whitespace only mirror
entries) there is no need to separate the entries in PREMIRRORS with
"\n".

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>