[3/3] Bump grpc from 1.26.0 to 1.31.1

PART 3: remove grpc 1.26.0 from third_party/grpc
Composed PR: bazelbuild#12226
Note: bootstrap jars & java_plugin remain to be 1.26.0 though

Fixes having external dependencies without checksum
- boringssl (each download was timestamped, but otherwise stable)
- bazel_skylark was overriden to be master (sic!)

There doesn't seem to be many breaking/big changes up to grpc 1.31.1
- removal of xds-experimental URI scheme
- removal of MAX_EPOLL_EVENTS_HANDLED_EACH_POLL_CALL
- enable TLS 1.3 in the C-core and all wrapped languages
- some of bazel-related patches got merged in
https://github.com/grpc/grpc/releases

How to check whether certain dependency has a checksum
bazel query //external:bazel_skylib --output build
bazel query //external:boringssl --output build

How to find (almost?) all problematic dependencies
compare output of
bazel query 'kind(http_archive, //external:all) + kind(http_file, //external:all) + kind(distdir_tar, //external:all)' --output xml | xq '.query.rule[] | ."@name"'
vs
bazel query 'kind(http_archive, //external:all) + kind(http_file, //external:all) + kind(distdir_tar, //external:all)' --output xml | xq '.query.rule[] | select (.string[]."@name" | contains("sha256")) | ."@name"'

Note that it looks for string sha256 and misses dict sha256 for
distdir_tar rules - those are false positive currently.

third_party/grpc/BUILD

@@ -18,7 +18,7 @@ load("//tools/distributions:distribution_rules.bzl", "distrib_java_import", "dis
 
 licenses(["notice"])  # Apache v2
 
-exports_files(["grpc_1.26.0.patch", "upb_gcc10_fix.patch", "grpc_1.31.1.patch"])
+exports_files(["grpc_1.31.1.patch"])
 
 package(default_visibility = ["//visibility:public"])