MB-9903 Remove old build instructions from README

Change-Id: I80e3cc10a2f6d1809deb9445f43aa2cf017434f1
Reviewed-on: http://review.couchbase.org/32369
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>