backport to 1.53: [promises] Make Poll<T> its own type, not a variant<> (#32540)

Backport of #32515 to 1.53.

Author: markdroth

BUILD

@@ -2339,7 +2339,6 @@ grpc_cc_library(
     external_deps = [
         "absl/status",
         "absl/types:optional",
-        "absl/types:variant",
     ],
     language = "c++",
     public_hdrs = [
@@ -3651,7 +3650,6 @@ grpc_cc_library(
         "absl/strings:cord",
         "absl/strings:str_format",
         "absl/types:optional",
-        "absl/types:variant",
     ],
     language = "c++",
     visibility = ["@grpc:grpclb"],