contrib/jenkins_manuals: remove old symlinks
In master-builds, the job runs without wiping the workspace. Remove
previous build and common symlinks, so the manuals job doesn't fail
with:
+ make -C doc/manuals
make: Entering directory '/build/doc/manuals'
ln -s /opt/osmo-gsm-manuals/build build
ln: failed to create symbolic link 'build': File exists
Change-Id: I88add52dc5a4671cf4dcfd498c3103023a35b7ba
