Revert "MB-10496: Add jemalloc as a cbdeps dependancy for CentOS 6"

At least some of the builders (e.g.
http://builds.hq.northscale.net:8010/builders/centos-6-x64-master-builder/builds/1188)
have a compiler which fails to compile some of the defragmenter code.
Reverting this, to no longer use jemalloc and hence not try to compile
the defragmenter code.

This reverts commit 062f83926a9a3d3649b6272f3667eaecc3ef9b71.

Change-Id: I5d4ad88e922770b62d5227e9a09b18a6758a848b
Reviewed-on: http://review.couchbase.org/44097
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>