Revert "Merge pull request #18093 from grpc/revert-17770-lb_policy_picker_api"

This reverts commit f327b8370652a14ac112be3c88ab08fcdf1c839a, reversing
changes made to b3b5d634231ce2c5c0ec0c557b6844e1a43b482e.
diff --git a/gRPC-Core.podspec b/gRPC-Core.podspec
index 92626f3..61409e9 100644
--- a/gRPC-Core.podspec
+++ b/gRPC-Core.podspec
@@ -354,11 +354,11 @@
                       'src/core/ext/filters/client_channel/parse_address.h',
                       'src/core/ext/filters/client_channel/proxy_mapper.h',
                       'src/core/ext/filters/client_channel/proxy_mapper_registry.h',
-                      'src/core/ext/filters/client_channel/request_routing.h',
                       'src/core/ext/filters/client_channel/resolver.h',
                       'src/core/ext/filters/client_channel/resolver_factory.h',
                       'src/core/ext/filters/client_channel/resolver_registry.h',
                       'src/core/ext/filters/client_channel/resolver_result_parsing.h',
+                      'src/core/ext/filters/client_channel/resolving_lb_policy.h',
                       'src/core/ext/filters/client_channel/retry_throttle.h',
                       'src/core/ext/filters/client_channel/server_address.h',
                       'src/core/ext/filters/client_channel/subchannel.h',
@@ -801,10 +801,10 @@
                       'src/core/ext/filters/client_channel/parse_address.cc',
                       'src/core/ext/filters/client_channel/proxy_mapper.cc',
                       'src/core/ext/filters/client_channel/proxy_mapper_registry.cc',
-                      'src/core/ext/filters/client_channel/request_routing.cc',
                       'src/core/ext/filters/client_channel/resolver.cc',
                       'src/core/ext/filters/client_channel/resolver_registry.cc',
                       'src/core/ext/filters/client_channel/resolver_result_parsing.cc',
+                      'src/core/ext/filters/client_channel/resolving_lb_policy.cc',
                       'src/core/ext/filters/client_channel/retry_throttle.cc',
                       'src/core/ext/filters/client_channel/server_address.cc',
                       'src/core/ext/filters/client_channel/subchannel.cc',
@@ -984,11 +984,11 @@
                               'src/core/ext/filters/client_channel/parse_address.h',
                               'src/core/ext/filters/client_channel/proxy_mapper.h',
                               'src/core/ext/filters/client_channel/proxy_mapper_registry.h',
-                              'src/core/ext/filters/client_channel/request_routing.h',
                               'src/core/ext/filters/client_channel/resolver.h',
                               'src/core/ext/filters/client_channel/resolver_factory.h',
                               'src/core/ext/filters/client_channel/resolver_registry.h',
                               'src/core/ext/filters/client_channel/resolver_result_parsing.h',
+                              'src/core/ext/filters/client_channel/resolving_lb_policy.h',
                               'src/core/ext/filters/client_channel/retry_throttle.h',
                               'src/core/ext/filters/client_channel/server_address.h',
                               'src/core/ext/filters/client_channel/subchannel.h',