Merge gtech_tool functionality into registry_tool

This change consolidates gtech_tool into registry_tool.  Since App Engine has
no actual ACLs on the remote API (any access is essentially root access), we're
removing this to avoid giving the impression to users that gtech_tool is truly
locked down from a security perspective compared to registry_tool.

In addition to merging GtechTool.COMMAND_MAP into RegistryTool.COMMAND_MAP, this
change also removes the {create,update}_sandbox_tld commands (which only made
sense for gtech_tool) and removes references to gtech_tool in the documentation.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=134828710

java/google/registry/tools/BUILD

@@ -96,13 +96,3 @@ java_binary(
     ],
 )
 
-java_binary(
-    name = "gtech_tool",
-    create_executable = 1,
-    main_class = "google.registry.tools.GtechTool",
-    runtime_deps = [
-        ":tools",
-        "//third_party/java/appengine:appengine-api-link",
-        "//third_party/java/appengine:appengine-remote-api-link",
-    ],
-)