Revert grpc upgrade

Due to a grpc bug breaking Bazel inside docer in Bazel 3.4.0, we have to revert the upgrade of grpc 1.26.0

Revert third_party/grpc/BUILD

This partially reverts commit 2ed3980.

Revert "Upgrade gRPC to 1.26.0"

This reverts commit dfbf87c.

Closes #11758

third_party/BUILD

@@ -47,6 +47,7 @@ filegroup(
         "//third_party/py/six:srcs",
         "//third_party/rules_python:srcs",
         "//third_party/zlib:srcs",
+        "//third_party/nanopb:srcs",
         "@googleapis//:srcs",
         "@remoteapis//:srcs",
     ],
@@ -487,16 +488,8 @@ distrib_java_import(
     name = "opencensus-api",
     enable_distributions = ["debian"],
     jars = [
-        "opencensus/opencensus-api-0.24.0.jar",
-        "opencensus/opencensus-contrib-grpc-metrics-0.24.0.jar",
-    ],
-)
-
-distrib_java_import(
-    name = "perfmark-api",
-    enable_distributions = ["debian"],
-    jars = [
-        "perfmark/perfmark-api-0.19.0.jar",
+        "opencensus/opencensus-api-0.19.2.jar",
+        "opencensus/opencensus-contrib-grpc-metrics-0.19.2.jar",
     ],
 )