metrics: Remove scollectd backend

It's no longer used. See
#2561 (comment)
and following.

Remove it which will allow simplifying some parts of the shared metrics
backend.

CMakeLists.txt

@@ -570,8 +570,6 @@ add_library (seastar
   include/seastar/core/rwlock.hh
   include/seastar/core/scattered_message.hh
   include/seastar/core/scheduling.hh
-  include/seastar/core/scollectd.hh
-  include/seastar/core/scollectd_api.hh
   include/seastar/core/seastar.hh
   include/seastar/core/semaphore.hh
   include/seastar/core/shard_id.hh
@@ -714,8 +712,6 @@ add_library (seastar
   src/core/reactor.cc
   src/core/resource.cc
   src/core/sharded.cc
-  src/core/scollectd.cc
-  src/core/scollectd-impl.hh
   src/core/signal.cc
   src/core/systemwide_memory_barrier.cc
   src/core/smp.cc

include/seastar/core/app-template.hh

@@ -31,7 +31,6 @@
 #include <seastar/core/sstring.hh>
 #include <seastar/util/program-options.hh>
 #include <seastar/core/metrics_api.hh>
-#include <seastar/core/scollectd.hh>
 #include <seastar/util/log-cli.hh>
 #include <seastar/util/modules.hh>
 
@@ -112,8 +111,6 @@ public:
         metrics::options metrics_opts;
         /// Configuration options for the smp aspect of seastar.
         smp_options smp_opts;
-        /// Configuration for the scollectd sub-system.
-        scollectd::options scollectd_opts;
         /// Configuration for the logging sub-system.
         log_cli::options log_opts;