MB-100: fixed build error due to bad .PHONY and rule interaction

I still cannot completely get why but change of ns_server.app.src
target to rule _and_ src/ns_server.app.src being presequisite of
.PHONY didn't work well. In this weird case which manifested itself
only when PRODUCT_VERSION is defined (i.e. under voltron), make simply
believed there are no steps for src/ns_server.app.src. Apparently in
this %.app.src rule was not considered.

Anyway, given we don't really need neither PHONY (.app.src depends on
TMP_VER which is .PHONY), nor rule (ns_babysitter.app.src has
hardcoded version so it's not built via sed), we can fix problem by
removing both sides of trouble.

Change-Id: Icd376d6679a86fde097340eefe1c863eb8dccfad
Reviewed-on: http://review.couchbase.org/25501
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>