CLEANUP: otel: move opentelemetry outside haproxy sources

CLEANUP: otel: move opentelemetry outside haproxy sources

The opentelemetry addons now live outside haproxy sources and is
available at https://github.com/haproxytech/haproxy-opentelemetry/

The addon must be built using the EXTRA_MAKE option from HAProxy
Makefile:

$ PKG_CONFIG_PATH=/opt/lib/pkgconfig make -j8 TARGET=linux-glibc
  EXTRA_MAKE="/tmp/a/a/haproxy-opentelemetry" OTEL_DEBUG=1 OTEL_USE_VARS=1