Remove collection of node gc stats in prometheus (#1545)

reasons for this

1. we have never documented the metrics
2. not sure how much use it is anyway
3. was using an old version of gc-stats which is now unmaintained and
has security issues
4. replacement version of gc-stats is maintained by a single person who
controls the binary builds available so is a dangerous thing to rely on
5. it has no pure javascript fallback therefore requires caliper to pull
down or build a binary module (meaning you need python and compilers
installed) which will also be dependent on the version of node being
used.

Signed-off-by: Dave Kelsey <d_kelsey@uk.ibm.com>

Author: davek-at-ibm