cmake: remove git fetch and use submodule for building opentelemetry lib

cmake: remove git fetch and use submodule for building opentelemetry lib

We were using git fetch and an unofficial remote to fetch
opentelemetry-cpp, as it needed additional patch to include boost
headers, with the merge of

open-telemetry/opentelemetry-cpp#1100
open-telemetry/opentelemetry-cpp#1020

we do not require to maintain our own patched version. Also, we were
using git fetch as a temporary change, now that tracing is always on, it
should be right time to adapt to using opentelemetry-cpp as a submodule.

This removes compile time fetch for opentelemetry-cpp instead use
official opentelemetry-cpp lib as a submodule.

Signed-off-by: Deepika Upadhyay <dupadhya@redhat.com>