deployment: remove declarations of obsoleted paths

Remove the declarations of /a/common, and update remaining references to
these paths. /a/common is now just a symlink to /srv/mediawiki-staging, so
replacing it with /srv/mediawiki-staging is a no-op. I will decom the old
symlinks manually. Also update Beta's scap staging dir to
/srv/mediawiki-staging.

Change-Id: I504269d784cc13599c0ab2e587b3f6ea1830c5ee