chore: Remove Cloud Debugger Backend support. (#75)

This change removes support for the Cloud Debugger Backend making the
Firebase RTDB backend the default. This change also renders the
`use_firebase` flag obsolete, though it is kept around to prevent
loading failures for invocations still setting it.

src/agent/internals/src/main/java/com/google/devtools/cdbg/debuglets/java/BUILD

@@ -65,21 +65,6 @@ java_library(
     ],
 )
 
-java_library(
-    name = "gcp_hub_client",
-    srcs = ["GcpHubClient.java"],
-    deps = [
-        ":gcp_debuglet_version",
-        ":gcp_environment",
-        ":gcp_metadata",
-        ":hub_client",
-        ":labels",
-        ":logger",
-        ":mapper",
-        "@maven//:com_google_code_gson_gson"
-    ],
-)
-
 java_library(
     name = "gcp_metadata",
     srcs = [
@@ -176,7 +161,6 @@ java_library(
         ":gcp_breakpoint_labels_provider",
         ":gcp_debuglet_version",
         ":gcp_environment",
-        ":gcp_hub_client",
         ":gcp_metadata",
         ":hub_client",
         ":labels",